Every time an image is compressed, it falls into one of two categories: lossy or lossless. The difference determines whether you can shrink the file more aggressively at the cost of some quality, or whether every pixel is preserved exactly at the cost of a larger file.
Lossless compression
Lossless compression shrinks file size by finding more efficient ways to store the exact same data — similar to how a ZIP file compresses a document without changing a single word inside it. When you decompress a lossless image, you get back pixel-for-pixel exactly what was originally encoded. PNG is the most common lossless image format.
Lossy compression
Lossy compression achieves much smaller file sizes by selectively discarding information the human eye is less likely to notice missing — subtle color gradations, fine texture detail in busy areas of a photo, and so on. This discarded data cannot be recovered once compressed. JPG is the most common lossy format, and it’s adjustable: higher quality settings discard less, lower settings discard more.
Why lossy compression can still look great
The human visual system is genuinely bad at noticing small changes in complex, detailed areas of an image (like foliage or fabric texture) but very good at noticing changes in smooth areas (like skies or skin). Good lossy compression algorithms exploit this by discarding data preferentially from areas where it won’t be missed, which is why a well-compressed JPG at 80–85% quality often looks identical to the uncompressed original in a casual viewing.
When each one makes sense
- Use lossless (PNG) for screenshots, logos, text-heavy graphics, and any image you plan to edit further — repeated lossy re-saves compound quality loss.
- Use lossy (JPG/WebP) for photographs and any final, published image where file size matters more than pixel-perfect preservation.
The generation-loss trap
A crucial detail: lossy compression loses a little quality every time you re-save, not just once. Opening a JPG, making a small edit, and re-saving as JPG repeatedly will visibly degrade the image after several rounds, even though each individual save looks fine. The safe practice is to keep a lossless master copy (PNG or your editing software’s native format) and only export to a lossy format as the final step, ideally just once.
If you’re not sure which format fits your situation, our resizer lets you preview and compare formats side by side before committing to a final download.