The maximum file size your profile picture can have is 0.15MB (150 KB). You can right-click the image/gif to view its properties, where you'll find its file size.
If your gif is too big, reduce the filesize of it.
To reduce the filesize of a gif you can:
- SCALE IT DOWN
Note, that using images/gifs bigger than 120x120 will unnecessarily take some extra file size, as the forums will reduce it to 120x120 anyway, due to its display.
- CROP IT
If resizing will make details barely visible, consider cutting some unused or less important parts. (dimensions-wise)
- CUT SOME FRAMES OUT (MORE ADVANCED OPTION)
The longer the animation is, the heavier the gif. Consider cutting out some frames at the beginning and/or at the end if these are irrelevant. If the animation is smooth, it would be possible to relatively seamlessly drop some frames in the middle as well.
GIMP would be more than enough to perform all of these changes.
If you'd have troubles, I could modify the gif for you if you'd like.