1) You're mixing an 8-bit background with... 16, 32-bit sprites? (most GBA games can go either way) It looks a little odd. I personally don't have a problem with it in this instance, but I know that some people will.
2) If 2 Goombas bonk their head together, don't you think they're going to look a little injured? I think you should've changed the facial expressions in the last 2 panels.
3) You shouldn't really save sprite comics in jpeg. They look really fuzzy when you do. Definitely go for either .gif or .png.