You've drawn, scanned, and colored your characters, now how do you get them into a freakin comic?! I've taken a bit of a shortcut for the tutorial by using a pre-made template I have for all the comics, but the idea is simple. Your trying to resize your images down so they don't take up five pages of html when people view them. Yeah, I know, you lose all those nice subtleties but no one cares if they have to scroll just to find Cortana's face.
Since a 300 pixel res image is so huge dragging them layer by layer into a smaller image can be a mess. Make life a little easier and merge the layers together before you copy the images in.