ok, the shrunk jpg that i reuploaded was not shrunk again (they probably recognize the hash of the file that they are already hosting). so it's at least good that they don't keep reducing the quality of JPG if they already have done it once.
interestingly enough i also tried with two PNG files above 4 MiB, one with DPI info in it (dots per inch) and one without, and BOTH where actually modified to be a little smaller and the DPI info was stripped.
so in conclusion i guess imgchest.com doesn't support lossless file uploading but still seems to be a good service. unless you can do that with an account? i've only tried uploading anonymously
note that the png QUALITY is still lossless, i know this because if i download their modified PNG and re-save it as a PNG and also re-save the original PNG to compare the resulting files i get the same SHA-256 for both. so even if the file you upload gets modified the actual content of the file does not (this is only true for PNG of course, not JPG, unless there's some magical new way to copy and compress unmodified JPG data that I don't know about).
tried uploading a 19.5 MiB gif image and it was also modified somehow, still 19.5 MiB so maybe they just changed the image metadata. i can't reproduce the same SHA256 however so i don't know if the gif quality is preserved without loss like png is.
tried uploading a 468 bytes gif image and they modified it too. into a 510 bytes gif image, it grew larger! so now we know that they ALWAYS modify your uploaded image, even if the end result becomes larger.
they don't support webp images, i think it's high time sites start to roll out support for that. but i'll admit that i havent really warmed to webp yet either, but it is a superior format and should be supported going forward for those that wish to use it. checked webm and swf as well, can't upload those (no surprise).