How Image Compression Works

Why image size matters

Large images create three common problems. First, uploads fail: email systems, forms, marketplaces, and support portals often have strict limits. Second, pages load slowly: images are usually the largest bytes on a webpage, so heavy images hurt performance and user experience. Third, sharing becomes inconvenient: sending a 10 MB photo over a mobile connection is slow and expensive for the recipient.

Compression is about reducing bytes while keeping the image visually acceptable for the intended use. The intended use matters because “acceptable” changes with context. A background hero image can tolerate more compression than a screenshot with small UI text. A product photo needs consistent color more than it needs microscopic detail.

Pixels vs bytes: what is actually being reduced?

An image has two different “sizes.” The first is its pixel dimensions (for example, 4000×3000). The second is its file size in bytes (for example, 6 MB). Pixels describe how many points of color exist. Bytes describe how those pixels are stored.

Resizing changes the number of pixels. Compression changes how efficiently those pixels are represented. You can have a small image (few pixels) that is still a large file (inefficient format), and a large image (many pixels) that is surprisingly small (efficient format plus good compression).

Lossless vs lossy compression

Lossless compression reduces file size without changing pixel values. If you decompress and re-open the image, every pixel is identical to the original. PNG (in its common usage) is a lossless format. Lossless compression can be very effective for graphics with repeated patterns, flat colors, and sharp edges. It is usually less effective for photographic content.

Lossy compression reduces file size by discarding information the encoder predicts you will not notice. JPEG is the classic example. Lossy compression is why a photo can shrink dramatically. It is also why over-compression causes blocky artifacts, ringing around edges, and banding in smooth gradients.

JPEG: why it works well for photos

JPEG is optimized for photographic images. At a high level, JPEG breaks the image into small blocks and stores an approximation of the image using frequency information rather than storing every pixel exactly. The “quality” setting controls how aggressively the approximation discards detail.

JPEG also relies on a human-vision trick: we notice brightness changes more than color changes. Many JPEG encoders reduce color precision more aggressively than brightness precision. For photos, this often looks fine. For screenshots and text, it can create color bleeding and fuzzy edges.

PNG: why it stays crisp (and why it can be large)

PNG is typically used when you need lossless output, sharp edges, and transparency. Logos, icons, UI elements, and diagrams are common PNG candidates. PNG can compress extremely well when large areas of the image repeat or when there are clean boundaries between colors.

For photos, PNG often becomes huge because photographic content does not repeat cleanly. A camera photo contains natural noise and micro-texture. Lossless compression cannot “throw away” that noise, so the encoder has fewer opportunities to reduce size.

WEBP: a modern alternative

WEBP is a newer image format that can be lossy or lossless and can support transparency. In many situations, it can provide smaller files than JPEG at similar visible quality. This is why many modern websites prefer WEBP for photos.

The main question with WEBP is compatibility. Most modern browsers support it, but certain older tools and platforms may not. If you are uploading to a place that does not accept WEBP, convert to JPEG or PNG instead.

Compression vs resizing: a practical workflow

If an image is used on a website, the most common mistake is serving a 4000-pixel-wide photo into a 1200-pixel container. You pay for the bytes even if the browser visually displays a smaller version.

A simple workflow is:

1. Resize the image to the maximum dimension you actually need for the target display.
2. Compress to reduce bytes while checking the preview at normal viewing size.
3. Choose format based on content type: JPEG/WEBP for photos, PNG for crisp graphics and transparency.

Tools to apply this workflow: Image Resizer, Image Compressor, and Image Format Converter.

How to choose a quality setting (without guessing)

Quality is not a universal number. A setting of 80 can be perfect for one image and unacceptable for another. Instead of looking for the “right” number, use an iterative approach:

• Start at a sensible baseline (around 85 for photos).
• Look at the preview at the size the image will actually be seen (do not judge only when zoomed in).
• Check the failure zones: text, edges, gradients, and skin tones.
• Adjust quality in small steps until the artifacts disappear or are acceptable.

If you need the image to remain sharp at high zoom levels (printing, design work, detailed inspections), compression is not the right lever. You either keep the original or use lossless settings.

Common pitfalls (and what to do instead)

1) Compressing a screenshot like it is a photo

UI screenshots have crisp edges and small text. Lossy compression can blur those edges quickly. If clarity matters, use a higher quality or keep PNG. If size must be smaller, consider resizing slightly instead of dropping quality aggressively.

2) Converting transparency away by accident

If a PNG has transparency and you convert to JPEG, the transparency will be replaced by a solid background. That might be fine for a banner, but not for a logo that needs to sit on different backgrounds. Use PNG or a transparency-capable WEBP.

3) Over-optimizing already optimized images

Some images are already compressed efficiently. Re-encoding them at high quality can create a larger file than the original. If the original is already small and looks good, keep it.

Metadata and privacy

Photos often contain metadata (EXIF) such as camera model, timestamp, and sometimes GPS location. Compression and format conversion might remove metadata, but it is not something to assume. If metadata matters, check it explicitly.

Use Image Metadata Viewer to inspect what is embedded, and Image Metadata Remover to strip metadata before publishing.

A quick checklist for smaller images that still look good

• Resize first if the image dimensions exceed the actual display size.
• Use JPEG or WEBP for photos; use PNG for crisp graphics and transparency.
• Start around 85 quality for photos and adjust after checking the preview.
• Look specifically at text, edges, gradients, and flat color areas.
• Keep originals for archive and future edits.
• Remove metadata intentionally if you are sharing publicly.

FAQs

Next steps

If you want to apply what you learned right away, start with the Image Compressor. If the output is still too large, resize first using the Image Resizer. If a platform requires a specific format, use the Image Format Converter.