or use this Quick Button
Select Forced library tab.

There is a situation: you want to make a mosaic for your office boss.
You have collection of 1000+ digital images with different office views, buildings, office desks, parking
lots, and also you have 104 senior office leadership photo.
You know that to produce a good mosaic you should use as many different tiles as you can. And you
also know that all these 104 senior office leadership photo must be in the mosaic. If you'll forget even
one of these faces your boss would be angry and will fire you!
Here is a solution:
Create two libraries with same settings:
A large one from 1000+ different digital images and
A small one from 104 senior office leadership photo.
Go to Forced tiles tab on the Manual tiles screen.
Open Small library with the Open base button, then press the Replace all button. All images from
your Small library will be placed into the mosaic.
Then close the base and close the Manual tiles screen.
Make sure that the Fix custom checkbox is on and render mosaic as usual using the Large library.
When you are finished, you'll get mosaic with all your 104 senior office leadership in the best suited
position and rest of the tiles will be rendered with other different photos.
Tip 1:
You may select an area of the mosaic where you want to insert forced tiles. Simply select the area in the
main screen by mouse drag over the image before press Replace all button. If Inv select checker on
the Main Screen is ON then forced tiles
will be inserted outside of the selection.
Tip 2:
If you want to insert the forced tiles into some specific region only you can use Mosaic Mask Tool
to
select the needed region.
Note 1:
If there are more tiles indexed in your "Force" base then needed to compile mosaic - only top the
needed number of tiles that will be used - i.e. if you want to make the mosaic 10 x 10 = 100 tiles and
indexed 150 tiles only the first 100 tiles would be used.
Note 2:
The result of this "force replacing" of tiles does not look as accurate as normal rendering. And is much
slower.
As in the example above - if you want make a 10 x 10 mosaic from 100 library - it would be better if you
render in normal way with repetition set to 1.
BUT if you want to make a 20 x 20 the mosaic and have 1000 different tiles, 100 of them "MUST BE" in
the mosaic and the remaining 900 "MAY BE" in this new feature. This will give you good results.
To preview tile rightclick on the image and choose Show file option. Current images will be shown in the
new popup preview window.