Another thread we had on the old forum. We start with one image, and then each new image edits the previous one in some way - add new elements, remove or replace things, or even make an essentially "new" image with the old one incorporated somewhere in it (although it should still be clear where the previous one is involved). We never switch to a brand new image - we just chain from the first one onwards. Feel free to incorporate anything from Google Images or the like - you don't have to create every part yourself, just do the editing!
Speaking of Google Images, I've taken the starting image from there:
