Honestly, I'm on the fence about it being just guest art. Personally, I would love it; I'd love to see another depiction of my characters. But as for something to post in lieu of a real update to the comic, I'd want something with more substance than just a picture of a character or two.
As a reader, I've never felt like filler art was very satisfying. I can think of a few exceptions, but overall its disappointing to go to a comic just see a character drawing.
Granted, its not like people are paying for this content, and when soliciting for this extra help it makes me sound quite ungrateful to refuse a submission. But I still just want to give readers something worth their time.
So as I said, I'm on the fence about guest art.
But if you had a funny joke to do as a comic, I wouldn't be bothered by a lack of backgrounds.
My webcomic: Mischief in Maytia