Collage-creating software

I have a college-sports photography web site, rod cannon.com, and am looking for a way to create collages on my computer that I can upload and sell as 20x30 prints.

PicMonkey doesn't seem to support collages that size. Can anyone direct me to software that will do this job for me? I want to be able to put 10-12 photos in a collage and have the resulting file be big enough to create as a 20x30 print. (Smaller, if necessary. But 20x30 is my ideal.)

I'm certainly no expert, but the way I'd approach this is to bring each photo into its' own layer in photoshop which would easily let each photo be positioned as necessary. The overall image size could be set to whatever you wanted it to be.

As suggested you can do the job in PS. The dedicated Picture Collage Maker Pro allows for huge output and makes the process pretty easy.

I have used Luma Pix FotoFusion for probably 10 years and I have made collages with up to 120 images and used to create 20 x 24's with 10 images for families of new born's.