A photo straight off a modern smartphone camera commonly weighs 3-8 MB. On a website, that single image can take longer to load than the rest of the page combined, especially for visitors on mobile data rather than fast broadband. Compression exists to close that gap: reducing file size while keeping the image visually close enough to the original that most viewers can't tell the difference at normal viewing size.
Lossy vs lossless: two very different trade-offs
Lossless compression (like standard PNG) shrinks a file by packing the existing pixel data more efficiently, without discarding any information — you get the exact original image back, just stored more compactly, but the size reduction is limited. Lossy compression (like standard JPEG) actually discards some visual detail that's statistically less noticeable to the human eye, which is why it achieves far larger size reductions but also why quality can visibly degrade if pushed too aggressively. Most everyday photos are good candidates for lossy compression; graphics with sharp edges, text, or transparency (logos, icons, screenshots with UI elements) usually hold up better as PNG or, where transparency and small size both matter, WebP.
Why file format matters as much as compression level
The same image saved as PNG versus JPEG versus WebP can differ enormously in size for comparable visual quality, because each format uses different underlying compression techniques suited to different kinds of image content. JPEG remains the most universally compatible choice for photographs. WebP, a more modern format supported by all current major browsers, generally achieves smaller file sizes than JPEG or PNG at equivalent visual quality, which is why it's increasingly recommended for web use specifically — though it's still worth keeping a JPEG or PNG fallback for older tools or contexts outside the browser that don't read WebP.
Where compression actually matters most
Compression has an outsized effect on two very concrete things: page load time and mobile data usage. A page with several uncompressed 5 MB images can take many seconds to fully load on a typical mobile connection, and every one of those seconds is a point where a visitor may leave before the page even finishes rendering. Compressing the same images down to a few hundred KB each, often with no visible quality loss, can cut that load time dramatically without changing anything else about the page.
Finding the quality level that doesn't look compressed
The goal isn't the smallest possible file — it's the smallest file that still looks clean at the size it will actually be displayed. An image compressed aggressively enough to show visible blocky artifacts around sharp edges or blurred detail in faces and text has usually been pushed past a reasonable quality setting. Testing a couple of compression levels side by side, at the actual display size the image will be used at, is a more reliable way to find that balance than guessing a single percentage and assuming it works for every image.
Compressing images without the guesswork
Use the Image Compressor to shrink files before upload or sharing while comparing the visual result directly. If you also need to change the pixel dimensions — not just the file size — the Image Resizer is a useful companion, since resizing to the actual display dimensions first often reduces file size as much as compression does on its own.