I think your collage looks great!
I would recommend Photoshop Elements and it does not have to be the latest version either. That would give you the most flexibility, but it would not really have templates or anything like that.
For that you would need more of a digital scrapbooking type software, I am not really familiar with them since I do everything from scratch. But I think scrapgirls.com has some good info on that and maybe even products. It's geared towards digital scrapbooking, but the basics are the same. If you have photoshop, I thought I saw that somewhere? Then scrapgirls does have tutorials I believe to help you use it.
I am not sure about the program you are using, never used it, but when I make my block style collages like you have done. I create a second layer, and cut out, using a marquee tool and my ruler and grid, boxes on the page and then can just plop my pictures in the background of all the holes.