If you want to make mosaic that contains all images from your library you may use quite simple two-step workflow.

Please note that the number of the tile images in the library must be less or equal to the number of cells in mosaic (otherwise it is impossible to use all pictures).

Here are the steps:

Now you have fully rendered mosaic with all images used.

Press Statistic button to see image statistic and verify that all images are used.

Few useful tip:

While all images are used at least once, but it may be not good if one image that was used only once is placed at very corner or very close to outward border (in this case it may be cropped if you'll frame the mosaic)

To avoid this situation you may want to apply render at step #4 to central part of the mosaic. Simply select central region with dragging mouse and start rendering process while region is selected before pressing Render button.

Also you may try to play with Rendering Sequence feature along with above mentioned two-step procedure. That way you may to render meaningless background with random order for first rendering and render significant areas of the main image with smart order for final rendering.