You could create a mosaic picture using different render modes:
Mosaic all - This option will start to render from top left cell down to the right bottom cell.
Random mosaic - This option will start a render of the mosaic in random order.
Smart mosaic - This option will start render of the mosaic beginning from the most essential parts of
main theme image. These parts are rendered when almost no tiles are used, so there are more left to
choose from in these critical areas. If you have less then 10,000 images in the library, it may help make
a better picture.
Usually Smart mode is good for all mosaic themes but if your image has large areas of plain color, e.g.
plain red logo on plain blue background, it is better to use Random mode.
If you don't need to render whole mosaic but only part of it you can use Mosaic Mask Tool
to select the
needed region and render only this region with desired render mode.