Running an 800×600 test photo (341.2 KB as a PNG) through this tool with "Keep original" format left it essentially unchanged — PNG re-encoding is lossless, so there's little to shrink. Switching the output format to JPEG at 50% quality and setting a 400px max width brought it down to 13.7 KB at 400×300 — a 96% reduction from the original. Most of that came from two separate effects stacking together: converting from a lossless format to a lossy one, and physically reducing the pixel count to a quarter of the original (400×300 has a quarter the pixels of 800×600, since both width and height were halved).
These two settings save file size in genuinely different ways, and combining them compounds the effect rather than just adding it. The quality slider controls how much visual detail JPEG or WebP compression is willing to discard within each pixel block — lower quality means smaller files but more visible artifacts, especially around sharp edges and text. Max width controls how many pixels exist in the first place, which affects file size independent of quality and also directly determines the image's display size. If a photo only needs to display at, say, 800px wide on a page, there's rarely a reason to keep it at its original camera resolution — resizing down first, then compressing, tends to produce a smaller file at better visual quality than compressing hard at full size alone.
No — everything happens locally in your browser using the Canvas API. The image is never sent to a server, which is different from most compression tools online.
PNG compression is lossless, so re-encoding a PNG rarely shrinks it much. For real savings on photos, switch the output format to JPEG or WebP — both trade a little quality for a much smaller file.
You can open JPEG, PNG, WebP, and most other common image formats, and export as JPEG, PNG, or WebP. Very old browsers may not support WebP output — if the download looks wrong, switch to JPEG.