It either generates a thumbnail from the content (usually the first frame of a gif as you can see with the black box) or you can make your own to represent it. There should be an 'update thumbnail' on the submission so if you just take a screen cap of a more 'middle' frame of the gif and upload that there should be fine. (and for future reference to upload with a thumbnail, it's under where you upload the image itself, before you get to the description and everything).

I tried both of those, first tried updating the thumbnail which didn't work and then tried just uploading it again and setting the thumbnail from the getgo, which also didn't work

I unfortunately have no answers from here then ovo;