I need help with printing photo collages

I've made several collages with different apps such as Diptic & PicFrame on my ip4s and I have uploaded them onto my desktop. I have attempted to print the collages through Walgreens however, the pics are usually cropped or don't fit no matter how I edit them using the Wlagreen photo editor. I would like to print these on 4X6 or 8X10. Any suggestions? Thanks.