When and how to use internet image formats Is an article that breaks down what are the advantages and down sides of JPEG, PNG, and GIF when used on the internet.
To try and sum up:
* JPEG:
o Good:
+ Photographs
+ Game screenshots
+ Movie stills
+ Desktop backgrounds
o Bad:
+ Windows application screenshots
+ Line art and text
+ Anywhere where fine lines or sharp color contrast is needed
* PNG:
o Good:
+ Text, line art, comic-style drawings, general web graphics
+ Windows application screenshots
+ When absolutely 100% quality is required (24 bit)
+ When alpha channel support is required
+ As a general replacement for anything that is a non-animated GIF
o Bad:
+ Photos, in-game screenshots (only when quality is not important and you're looking for small files)
+ Disappointing browser support from Microsoft and others
* GIF:
o Good:
+ Where animations are absolutely required
+ Widespread browser support
o Bad:
+ Patented, legal techicalities
+ Large file sizes compared to PNG for the same quality
+ Obsolete
The AndreaMosaic program allows you to make one of those photo mosaic where the larger image is composed of many smaller images.
Of course the more images the program has to choose form, the better the results. Definitely one to experiment with.
The license under which the software is distributed is not mentioned but seems like freeware. The site also contains tutorials for most topics.
The Rastorbator is a web service which creates huge, rasterized images from any picture. The rasterized images can be printed and assembled into cool looking posters up to 20 meters in size.
The site outputs multi-page PDFs that are them printed and assembled by hand. To achieve such a huge size form a small images, the pictures are rasterbated: each pixel is magnified and separated. This means the images don't look good up close, but is perfect for poster sized images