Hi Nicki --
This might be old, old news by now, considering your request for help came in last May... but I found your thread as I was trying to locate info on a very similar issue we are experiencing, and I wanted to make you aware of it, at the least.
I've done a great deal of testing and sleuthing attempting to make our issue reproducible for the Jive folks, and I put in a Case with Jive support a short while ago.. I just went back and made it public (you can have a look here: https://community.jivesoftware.com/thread/166660 -- you should be able to access it now that I have made it public).
My Case has to do with the same thumbnailing you have seen occurring with your images. In short though, any image from 1 to 310 pixels wide will be untouched by Jive code and will be seen in your final published docs at full resolution; images from 311 - 620 pixels wide will be thumbnailed to 310 pixels wide and will need to be clicked up in order to see the true size; and then, as a kicker, all images from 621 to MAX pixels wide (whatever your largest width image is that exceeds 621 pixels) will be thumbed to 620 pixels wide, and must also be clicked on to see their full size. Of course, if you have an image that is super-wide, the Jive graphic pop-up window will not exceed the bounds of your browser window, so will be down-rezzed to fit within its boundaries (of course). You can read all the gory details in my Case and in the various associated documents I've assembled over there.
On top of all that, we exposed a bug in Jive code where server settings made by admins regarding thumbnailing sizes are being totally ignored (so the whole 310/620/MAX thing will eventually be able to be set my us and have those settings honored instead of having to go with Jive defaults).
Hope this helps.