How do I scale an image's print size? Answered

I'm printing some images that need to be a specific size. I've been using Gimp to edit them, and Gimp allows you to scale down using inches, but I'm getting different sizes from each image even though their all supposed to be the same size. I'm assuming this has something to do with the images resolution, but I'm not very graphic design savvy. Also, the images are being saved as pdfs. Any suggestions?