I hope others will chime in, but I can share a little about replacing third-party images at U-M.
Your question about how much to mimic an original image in creating/drawing/finding a replacement is a very good one. At U-M, we try to look more at the meaning of the image (what about the image is important to the instructor's example?) rather than the specifics of an image itself. For example, in the diagram you mentioned, without seeing the images you mentioned, I'm guessing that the instructor was trying using the Smarties box because they are round, small, and possibly stacked. The only reason he used Smarties is probably because students are familiar with that brand, but other round, small, stacked objects would also work for the instructor's example (and may even be better for the example and more visually interesting). There are a couple options:
1) You could actually buy a pack of Smarties and take a photo. Since you took the photo, there are no copyright concerns with the image. Copyright is different from trademark (ie brands). Yes, Smarties are trademarked, but since you are using the photo to refer to actual brand, there's no confusion or dilution (the two tests for using trademarks), that's ok.
2) Search or draw a replacement that incorporates small, round, stacked objects. Something like a stack of poker chips, cookies, potato chips; jars or bins of various candy; etc. You could also use insert-Shape in PPT or look for open clipart (like http://www.clker.com/search/round/1
) of a single 3D circle and then copy-paste to create a stack.
If I were you, I would do #2.
I hope that's helpful!