Yeah good point, there are a lot more IMG editors out there, it's been a wile since I modded an IMG file for SA but I remember using a program called GImgIO. Don't worry if it sounds Japanese, cause it is Japanese. Back then it didn't have an English version and IDK if it has one now but it you're used to working with IMG Tool 2.0 you'll know how to use it.
What I liked about GImgIO is that you could drag and drop as many files as you wanted, it didn't require archive rebuild or save and it would automatically sort out the files (especially useful for installing radar / map reskins).
The old link doesn't work so you might have to Google it, but here it is anyway:
http://aquila.ddo.jp...492/?q=node/273
EDIT: New link, still in Japanezie tho http://aquila-develop.net/GImgIO