When doing Jive and Social Collaboration consulting for teams and organizations I noticed that people encounter issues with graphic sizes as it relates to Jive Documents, Document Viewer Tiles and images in tables. In our cloud instance I have found that Jive does adjust images for the most part. The following seems to work for me, give it a try.
Add a Banner to a Jive Document - 1265 x 250
Try the following:
Gif Animation - Demo - Click image to view larger