Resizing or putting other image parameters to a GIF image might result in the following error in the log after the resized GIF is viewed in a browser:
13064 08:42:09 ERROR Could not run the 'getMediaStream' pipeline for '/sitecore/media library/web'. Original media data will be used.
Exception: System.ArgumentException
Message: Parameter is not valid.
Source: System.Drawing
at System.Drawing.Bitmap..ctor(Stream stream)
...