collage maker for mobile wallpaper

I want to make wallpapers for my iPhone by putting together my fave pics in a collage. But i cant seem to find a free online one that works for me. Most of them make the collage but then it doesn't fit my screen. I need one where i can set my own resolution and size for my template and then be able to crop pics and everything within the collage maker. Do you know the AmoLink photo collage maker? I heard that it can make mobile phone wallpapers. Any ideas of one like that? Thank you.