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Understanding image compression

How image compression works

Image compression comes in two flavors. Lossy formats such as JPEG, WebP and AVIF discard details your eye rarely notices, trading a small amount of quality for a large drop in file size. Lossless formats such as PNG keep every pixel intact, so the image is identical after decompression, but the files stay much bigger. For photos, lossy compression is almost always the right choice.

In JPEG, the quality setting controls quantization: how aggressively color values are rounded before the image is encoded. Lower quality means coarser rounding and smaller files. JPEG also applies chroma subsampling, which stores color information at lower resolution than brightness. That is why very low-quality JPEGs can look blurry or show color artifacts around edges, even when the shapes themselves look fine.

PhotoReducer does not use a basic built-in encoder. It runs MozJPEG for JPEG, libwebp for WebP, libavif for AVIF and Oxipng for PNG, compiled to WebAssembly. These are the same mature, highly optimized encoders used by professionals, and they consistently deliver a much better size-to-quality ratio than a simple encoder at the same quality setting.

Which quality level to choose

Higher quality numbers keep more detail but produce larger files. In practice, the best value is usually between 75 and 90: you keep most of the original quality while the file shrinks noticeably. Below about 60, visible artifacts become likely, especially in flat skies, gradients and skin tones. The table below gives typical ranges; your exact results depend on the photo.

JPG, PNG, WebP, AVIF or HEIC: which one to use

Each format has a different trade-off between file size, quality, transparency and compatibility. JPG is the universal default for photos. PNG is lossless and great for graphics with text or flat colors. WebP and AVIF are modern formats that compress photos much better than JPG, with full transparency support. HEIC is the iPhone default, but many websites and desktop programs cannot open it, so converting to JPG is usually the safe move.

Why it matters that processing happens in your browser

Some photos are sensitive: ID documents, contracts, children's faces, medical reports. When you use a tool that runs entirely in your browser, those files never leave your device. There is no upload, no server copy, no queue behind other users' files. With PhotoReducer, the only place your photos ever exist is the memory of your own computer.

There is also a speed advantage. Online compressors that upload to a server add upload time, processing time and download time, and they often impose limits on file size, number of files or daily usage. Because PhotoReducer works locally, results appear almost instantly, you can process as many images as you want, and it keeps working even when your connection is slow or flaky.

How to use PhotoReducer in three steps

The whole workflow takes seconds. Pick your photos, choose what you want to change, and download the result. No account, no waiting.

Common file size limits

Many forms and platforms impose strict size limits on image uploads. When you know the limit, use the compress-to-exact-size tool: PhotoReducer tries quality levels and only reduces dimensions if compression alone cannot reach the target. The table below lists typical limits you will encounter.