Images should be no problem if you generically number name them in your .fla (picture1.jpg, picture2.jpg etc... ) and assuming they (the client) don't upload progressive .jpgs, which Flash can't handle, when updating the pictures on the server. Dimensions of those pictures might be a concern, and you maybe should make sure they stick to dimensions you specify.
Updating text files, especially if they were to be html formated, might cause more serious problems though. Only a few specific html1.0 tags are supported by Flash, and the use of some special reserved characters such as the "&", could even keep the text from being displayed properly.
I think the person reponsible for updating should be aware of those restrictions...