Hi there,
Thanks for writing in, Im sorry to hear about the printing troubles. Each photo has what is called an aspect ratio. If you want to be able to print your photo at 4x6 or 8x12 you first must crop the photo to this ratio before sending it to print. If you change the pixel dimensions without cropping, this will distort your image.
If your images are at the 5x7 ratio to start, they will have to be cropped to fit the 4x6 ratio. If you are able to crop the photos yourself, this will allow you more control over where and how your photo is cropped.
Once you have a crop you like go ahead and save the photo and it should be ready for print

I hope this helps!
Cheers,
Chelsea, Premium Support Team