Lossy vs lossless compression
Lossy compression discards details the human eye barely notices to shrink file size, at some cost to quality. JPG and lossy WebP fall into this category.
Lossless compression loses no pixel information and shrinks files by optimizing storage. PNG and lossless WebP belong here, but the room for compression is limited.
Compression traits of the three formats
JPG: suits color-rich images like photos. Quality 80–90 usually shrinks size dramatically.
PNG: suits screenshots, logos, and icons that need crisp edges, but compresses photo-like content poorly — such files barely shrink.
WebP: supports both lossy and lossless modes plus transparent backgrounds, and at similar quality is usually smaller than both JPG and PNG.
How to choose the quality parameter?
The default 80–90 is the balance point between size and quality and suits most cases.
For quality-sensitive work (print, design files), choose 95 or above; for size-sensitive cases (web thumbnails), drop to 60–70.
After conversion, the tool shows the sizes before and after plus the savings percentage, so you can fine-tune the parameter based on the result.
Why is the file still large after compression?
Image dimensions (pixel count) are the main driver of file size. A 5000×3000 photo can stay larger than an uncompressed 800×600 thumbnail no matter how hard you compress it.
If compression results are disappointing, use the resize tool to shrink the image first, then compress — the combined effect is usually much better.