Unfortunately I can't fix the 'flatness'. The original source has disappeared from the Internet ether.
You're right that you cannot fix the display directly (tacit might), but you could do something at your end. Your avatar .gif is fine by itself, but (like Virtual1's) displays incorrectly in the forums because of its pixel size,
yours being 51x116 px and
Virtual1's 47x104 px. It's not because it is an animated gif:
mine displays fine at 70x70 px even with the previous 80x80 limit.
You can easily see that for yourself if you save the file (right- or Control-click on the avatar and select 'Save Image To', or simply drag it to the desktop). Then open an empty browser tab or window and drag your file ('73.gif') into it to see the proper dimensions with its pixel size displayed in the tab/window title. Btw, this also shows that the original source of the image is no longer necessary because you already have a fully working copy.
Even so, one of these days you or your kitty may choke on OS 9...
