1) I don't use an external image editor because sometimes I want to use GM's built in one, and sometimes I want to use an external editor. It's annoying to have to go hunt down that option every time just to switch it up. The ideal would be to have, say, 2 sprite editor buttons, maybe have both of them configurable (e.g. one as the internal, and one as a specified external -- or both as different externals).
2) I don't use an external code editor because it has no way of picking up resource names and the function list for highlighting. I would love to be able to, because I can think of quite a few editors that add a number of awesome features that GM's lacks, but I just can't use them without resource name and function highlighting.
Also, if you have to pause GM while the editor is going so you know when editing is stopped, you might consider an alternative method which LateralGM uses: a threaded file listener. I know this introduces some problems (when to stop listening, what to do if the image/sprite is modified, etc), but LGM has figured a way around them and implemented it pretty well. You might take a look at their source code (it is open source) and get some ideas (no direct code porting, though, as that is license infringement).
Edited by IsmAvatar, 24 January 2011 - 08:51 PM.