Collagerator is a free tool which makes it very easy to create professional photo collages from your favourite digital images.
The process starts with some basic configuration options, like choosing your preferred collage design (background colour, how the images will be arranged), its size and orientation (portrait or landscape). Add a few images and they'll immediately be organised according to your rules.
This probably won't be exactly right, but the design can be tweaked in various ways. Right-clicking an image displays options to move it above or below others; you can drag and drop a picture wherever you like; and the Theme panel allows you to change the background colour, your image frames, shadows, or just create a new and random arrangement at a click.
Some of these options can get quite involved. In particular, photo shadows can be assigned a custom colour, angle, size, blur radius and more. It can take a while to set everything up precisely as you'd like, then, but you only need to do it once: Collagerator allows you to save your configuration as a "preset" which can easily be recalled later.
And once you've finished, the program can export your format in various image formats (BMP, ICO, JPG, PNG, TIFF and more), with your preferred width, height and DPI settings.