How do you explain pixel dimensions?

I'm working on a web script for a dog rescue group my wife works with.  Every year the print a calendar of photos submitted by people that have adopted dogs through the group.  Last year my wife was handed the project along with all the emails that had been sent in, and the problem was that most of the images were too small to print well (lots of cell phone shots).  So what I'm doing is having the script check that each image is at least 1200 pixels along each dimension so that at 8" it'll be 150dpi which is about the minimum I think looks good printed. 

But how do you explain that to non-computer people?  Would it be clearer if I said "at least a 1.5 megapixel image"? 

I think that would be the best way to explain it.  1.5 pixels isn't very big though, is it a small calendar?

Even less thechie than "1.5 mp", just say you require "sharp" pictures and that mobile phones aint good for that

What we did last year was to take any we really liked but were too small to be their own page and included them in a "honorable mention" page at the back.  A 1.5" image doesn't take much resolution. 

The problem was last year people were just emailing the images as attachments so they sent them in email resolution and we got images that were 320x240.  Tough to print from.