I've never had iResizer crash on me, so strange to hear it did for you. Though to be honest any image manipulation app should allow you to resize and image so it shouldn't be difficult to do.
That aside, well done with figuring out the background needs to be place to 0,0 for it to be drawn from the top left corner. I agree that the values in the theme.plist can be confusing at times when it comes to using a % or not.