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I am sure you can...go to site admin, click on xcgallery, go to general configuration and look for Quote:
Accepted file extensions for uploaded pictures GIF/PNG/JPG/JPEG/TIF/TIFF
is what is in mine...maybe your lost your GIF somwhow??? I haven't touched my allowed types and it is first on the list....however you can ad to that list - perhaps those images not working had a slight different .filetag after them eg pjpg (progressive Jpeg??) because some OS and programs create their own little tags rather than the standard jpg or jpeg. There was another thread on this in the last day or so...
hereCheers