first off, you definitely wanna be the one uploading your image as a GIF that doesn't hit the file limit, if you either exceed the file limit, or upload in a different format, the site either rejects your upload, or handles the conversion itself, and they'll prioritise saving their disk space and bandwidth, and compress it real hard, more than it probably needs to if you spend the time editing a 100x100 thing yourself in a photo editor

you wanna use a photo editor for this, something at least that gives you a bunch of exporting options for GIF. from there, just experiment with the algorithm used for the particular GIF, the dithering level and etc. there isn't necessarily a need to understand the terms, but just to fiddle and see what looks right, and what gets you under the file size limit without compromising too much quality

when shrinking down something too, consider that you might need to have a more aggressive crop than usual, as details gonna be lost, or if there's a lot of colours, you're gonna end up dealing with GIFs limited palette of 256 colors (it really is a stinky format). if you want more defined but crunchy lines when shrinking or expanding something, then you could use nearest neighbor sampling, otherwise, bicubic tends to be the best bet in most cases (even if edges become a tad fuzzy, it tends to be noticeable here)

those are some general tips, basically, you wanna make sure you're the one uploading a GIF that doesn't hit the file size limit, so it doesn't get compressed by the website during upload

Thank you for the tip! 👌🏿

Shame this site would rather have you jump through hoops just to get a decent pfp.

they're fine with updating the ancient file size and image dimension limits on here, but only when it benefits them of course