In Evolution, I can open a pull down menu to select an application for opening attached image files. Since I have Gimp installed, it occurs on this list. How can I remove this option from the drop down menu (I never want to run Gimp in this way, but I sometimes accidentially hit it)?
I've looked through Edit->Preferences but didn't see anything that looks like I can configure what applications are presented.
That's nothing you configure in Evolution. Applications are rather registered globally with your system. But be aware that making it "unavailable" will be a global thing: you can either "unregister" Gimp from a given mime type generally or not at all (sure, that wouldn't keep you from open it manually).
Not that's clear where the association lies, I don't want to re-invent the wheel. For how to deal with them, please take a look at
You're lucky, it even uses Gimp as example :) For more details on those "associations", you might wish to follow up to our mime-type tag and browse its questions.