You may switch rendering modes at Preferences screen

You could create a mosaic picture using three different render modes:

Smart mosaic mode - 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.

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

From Top To Bottom mode - This option will start to render from top left cell down to the right bottom cell. 

Tip:
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.