To add to this, I’d recommend creating an image(or using the one provided in Fm_Trick’s post) with a circle and outline(transparent or any color works really, I prefer transparent though) so you can see exactly how it’ll look when you upload it.
I would recommend going with a bit more resolution than the previously mentioned 150x150 pixels. It might show up that size on your computer, but it’s nice to have it upscale well in case Roblox ever decides to (or when you want to show the icon in game, at a higher resolution).
(Unless Roblox automatically compresses the image to 150x150, like they do on regular decals with 1024x1024. If that’s the case, you are better off uploading in 150px. But I’m not able to test that right now.)