I did a tutorial a while back for the Eurobricks Reviewers' Academy on how to do this, and how to take it that step further, and add a CGI background. It covers Photoshop, Jasc and Gimp, although the main program described is Paint.net, which is free, but - in my opinion - is better equipped than Gimp. Unfortunately the Academy is a private forum, although you can gain entry just by requesting it, no questions asked.
However, for easy reference when needed, I also posted a similar topic at the same time publically, but locked - you can read it here
. It should explain everything - just ask if there's a problem. As Bruce said, a white sheet makes editing much easier - or not even necessary!
Bruce N H wrote:Here is an article on GIMP
, a free program you can download that does (as far as I can tell) everything that photoshop does.
Not by a long shot, although it does the job just fine, only much slower