Jaime, If I needed to make a picture shorter with the rest long, I'd just fill a row of brads, or maybe stitch a bit of paper down to bring the short photo up to the same length of the longer ones.
Here's a for instance.
Project 365 - May - first Saturday - Digital Scrapbooking Ideas - DesignerDigitals
I wasn't going to be able to make those two photos any longer or wider so I just filled in the gap with brads.
Or another for instance.
Project 365 - March 8 to 14 - Digital Scrapbooking Ideas - DesignerDigitals
Filled with a stitched bit of paper.