Render modes

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.
 Mosaic all.

Random mosaic - This option will start a render of the mosaic in random order.
Random mosaic.

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.
Smart mosaic.

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.

See also:
Preferences screen
Mosaic Mask Tool
Mazaika - (C) 2000-2018 by Boris A. Glazer.