When this plugin is enabled, you can get a thumbnail of any picture by adding a suffix to it path (requires URL rewritting).
For example: http://www.domain.com/path/image.png :
Thumbnail with fixed dimensions : http://www.domain.com/path/image.png.mini.100x100.jpg
Thumbnail with fixed dimensions keeping scale : http://www.domain.com/path/image.png.crop.100x100.jpg
Thumbnail with fixed dimensions keeping format : http://www.domain.com/path/image.png.fill.100x100.jpg
Thumbnail with maximum dimensions keeping format : http://www.domain.com/path/image.png.limit.100x100.jpg
Be careful, transparency of GIF/PNG is not preserved.