When I try to upload a vertical image I always get an "image is too large" error, but if I do anything horizontal it works just fine. I'm using Adobe to try and resize the photos, but they already say 1200X1600, which is what is suggested fo size. What size should I set this at in Image resize in Adobe photoshop elements? Do I look at pixels or inches?

that could be your problem. 1600x1200 is what you want. 1200x1600 is the wrong way round.

the website will only accept a horizontal image .. so you'll have to resize to 1200 vertical dimension and float it in a sea of black (or whatever)

similar to .. this