Yeah, editing them will be a bit tricky if you don't know what you're doing. If it's an RTP sprite or the cell height is only high enough for the character with no gap above, you'd need to increase the height of the spritesheet, and then move each row to space them out correctly, before putting in your bubble.
A script that would place a bubble above their head permanently, which you could turn on and off via a script call or a comment in the event, would be the easiest for you to use, and would not require a second copy of every NPC sprite you want a quest from.
I will see if I can write a script for you - in fact, I kind of remember doing something very similar for someone else once, so I may already have one here.
Question ... do you want it animated?
edit: in fact,
here you go
Also, don't double post. If you want to reply to several people and quote their posts, use the MultiQuote button.