First of all, giving credit is important, but starting a comic with sprite credits is a bad idea. Making it a news post or putting it in your commentary makes more sense.
But, let's take a look at that 2-panel comic.
#1: There's no background! Go get some backgrounds from http://bghq.com/ !
#2: There's no text bubble! Simply put a box (filled in with white) around what a character says and make a triangle at the bottom pointing at the character's mouth.
#3: That boulder is PATHETIC! Get some boulder sprites. Or at least color it in! No, a circle does not make a boulder.
if you rush a comic then what's the point in even bothering to put it up if you don't want to make it look good, don't waste your time and don't waste ours.