Omg Vista Print! I do all mine I'm there. The quality is amazing and they are cheap. Can't recommend enough.

Even just try Microsoft publisher. Probably the easiest. Can get pictures online and save them and insert into document, can do what eve text/design you want. There are templates to work off if want or just do your own.

Try Vista print. You can download it to you email. So many cute invites

I make birth announcements, birthday invitations etc etc etc. I use a mixture of programs but find Pizap the easiest for invites. :)

Maybe a free online photo collage site?

That's what I'd use...can add pics stickers and text boxes and save as whatever file you want