This extension uses ImageMagick for image resizing. That can be made to work with GIFs but it requires some extra steps: https://askubuntu.com/questions/257831/ ... magemagick
However, it looks like the extension utilises phpBB's existing functions for ImageMagick, so any changes to resize animated GIFs would need to be made to phpBB core, not the extension. Does ImageMagick manage to successfully create animated thumbnails for animated GIF attachments? If not, the problem is with phpBBB and the solution is therefore to submit a ticket on area51.