Here's an example:

bedroom.jpg: 250 x 350 pixels --> 15,229 bytes

bedroom2.jpg: 250 x 382 pixels --> 15,867 bytes
Thirty-two more pixels gives you two more lines of dialogue, at least on my system. I little experimentation showed that I was able to enlarge the panels twice that much by filling a dialogue box, but the gain in file size still wouldn't be bad if the backgrounds were extended sixty-four pixels. I wouldn't worry about people extending horizontally or filling both dialogue and narration, as that happens very rarely.
Also, I had to guess at the quality of the compression for the JPEG, so I'm not sure if I upped the compression when I did this. It doesn't seem to have compromised the quality of the image, though.
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Life is a lot like getting mugged; you get your kicks, you take your punches, and when it's over, someone else gets your cash.