My friend Katherine introduced me to some awesome storyboard templates from a company called GingerPixel. They are basically a set of PSD files that you open in Photoshop, add/arrange photos and save. The full instructions on downloading and using the templates can be found HERE. It can be a little time-consuming, but it is the easiest tool I have used to create collages.
There are eight different arrangements to chose from.
Collage templates from Ginger Pixel [http://www.gingerpixel.com/]
Below is a “Red” themed storyboard that I created using the Nine Square Grid template. The templates all have a white background. I played with the background color after flattening the file to a JPG image.