Mosamic is an excellent free tool which creates photo mosaics (pictures made with the tiny thumbnails of other images) from your own digital photos, or Flickr images.
The program is designed for extreme ease of use. Launch it and a tiny start screen appears, with just four buttons. "Create a new mosaic" appears at the top, making it a very obvious choice, and clicking the button launches a wizard to walk you through the process.
This starts by choosing the image you'd like to convert into a mosaic, then your preferred mosaic type. Mosamic provides two options here. "Colour Mosaic" fills each tile with solid colours, creating a pixelated effect - not very interesting. "Photo Mosaic" is much better, as this builds the final picture with other images.
Click "Photo Mosaic" and you're able to choose the number of tiles to use. Mosamic draws a grid over the image representing the current tile size, and updates it as you drag a slider, helping you get a feel for how your setting will affect the final image.
If you know what you're doing then there are several "Advanced" settings to tweak, like whether tiles are allowed to appear twice in the same mosaic. In our experience these can be safely left alone - the default settings are fine - but they're here, if you need them.
The final step is to add the images which will be used to build the mosaic. Just as with similar tools, Mosamic can import the contents of a local folder. But more unusually, it can also run a Flickr search on your chosen keyword, and download up to 1,000 images (which is much faster than you think, as it only needs the thumbnails).
This Flickr support is very useful as a quick way to find photos, but that's not all. The results for every search are saved for reuse later, with their keywords, and that presents all kinds of interesting possibilities. Download 1,000 "beach" images, say, and you'll know that these will emphasise blue/ white/ yellow. This could make them a great choice for photos with a similar colour palette, or you could deliberately pick them for something very different - a picture of a forest, say - just to see what happens.
Once you've added to your tiles library, click "Create mosaic image" and Mosamic builds and displays the finished mosaic for you. If it doesn't look good, you can click "Back" and try again; otherwise you're able to save the results as a JPEG, PNG or XPM file.