To combine several images into a film strip (not required, but it has advantages) I’ve tried several apps, and these are the best I’ve found so far.
Mac:
Image Stitcher (.99 cents)
https://itunes.apple.com/gb/app/image-stitcher/id819501444?mt=12
The author is very approachable, and actually added BMP to the export options for me. Unfortunately, it uses the standard Mac image library which stupidly encodes BMP bytes in reverse order. So, while this tool does a great job stacking images, it won’t play more than the first frame on Game Frame until you open the file in another App (like Photoshop) and save it out again (24 bit). I haven’t found an alternative to Photoshop on the Mac that saves the byte order correctly, but I haven’t spent much time looking yet. Would love some help here.
PC:
GlueIt 1.06 (Free)
http://www.varcade.com/blog/glueit-sprite-sheet-maker-download/
Super simple, works great. You want a single-column image. It saves a 32-bit BMP though, so you need to open the exported file in another App (Photoshop or Irfanview work well) and save as a 24-bit BMP.