This is screen shot of DesignLabComposition © Dominic J. Melfi author/programmer.
There are 2 images overlayed, foreground lake background Flag. (Any number of overlays can be managed)
program is working with reduced resolution workfiles so is very fast (Instantaneous).
When ready for output you generate the size image you want and the progrsm applies all positioning, sizing relative to composition, color and rotation and transparency to larger image.
Right now it will produce up to 8192×8192 output. If you have a collage of 2000×3000 images this will maintain a very good resolution for print output up to 34×34 inches.