Collage construction isn't well-suited to most image editors. Sure, you can resize and reposition a dozen photos yourself, but what if you want to create a collage using hundreds of photos? cf/x software's alpha 1.2.5 is designed specifically for this task, helping you to create attractive photo collages in a number of clever styles. (The software is available in additional configurations: alpha home, an entry-level edition sells for $89; the upcoming alpha pro, a scriptable edition that includes templates, will sell for $389.)
cf/x alpha handles photos much like an illustration application. All edits are non-destructive, meaning that the fidelity of the original photo is maintained no matter how often it is mutated. You can crop, scale, rotate, and reposition the photo however you wish; you can also group together photos just as you would group objects in most applications.