The reason you can't use this is because, since you're not a donor, you can only have a 100x100 PX avatar AND said avatar cannot exceed a file size of like... 50mb I think?
Anyway, this one that you linked to can't be used because it's too tall. If you have a program that edits GIFs, you could probably square it up so it's about this size:
And delete some of the frames so it's within the size limit.