He wants to view two pictures side-by-side.
Actually, it's less that than having a consistent way to switch from document to document - alt tab, to be specific. Yes, I know about ctrl-tab or whatever it is, but it's still not alt-tab.
I know it may seem obnoxious or naive to say "it seems easy," and though I'm not a Windows programmer, I've done more embedded stuff, and I have at least an idea of what features could probably be easily left undisturbed and which would hopefully be a very short list. When you have a program that prevents multiple instances, does it actually start to run a second instance and then hand off the document to the first instance and shut down? If so, that would "probably" be a very easy fix. The overwhelming majority of MDI and the rest of the code remains untouched (assuming that is how the multiple instance blocking works).
I can't for the life of me figure out why people want different document switching conventions for different types of documents, but to each their own. I just appreciate it when you can address 2 different approaches and satisfy everyone, as Firefox has done since they added tabs (bleck).