Shrink PNG screenshots and graphics
PNG is a lossless format, which is great for screenshots, logos, and graphics with sharp edges or transparency — but it also means PNG files don't compress the way JPEG does. There's no quality slider that throws away detail; a PNG either keeps every pixel exactly, or it doesn't.
Because of that, the quality slider has no effect on a PNG — the main ways to meaningfully shrink one are to reduce its resolution with the Size controls, or to give the tool a target file size in KB and let it search for a resolution that fits, which reduces file size without introducing the blocky artifacts you'd get from a lossy format.
If your PNG is actually a photo (rather than a screenshot or flat-color graphic) and you don't need transparency, converting it to JPEG or WebP will usually shrink it far more than staying in PNG ever could, since those formats compress photographic detail much more efficiently. Downscale Image lets you pick JPG or WebP as the output format right alongside the compression settings.
As with every tool here, compression happens entirely in your browser — your screenshots and graphics, which can often contain sensitive on-screen information, are never uploaded anywhere.