
WidsMob Montage is a Mac app (program) I discovered when it briefly became available as a free download instead of its usual price of up to $19.99 This is what I will be using for the next set of puzzle pictures starting a week on Monday under the title Monday Montage.
This is how it works.
First choose the background photo, in this case a Robin:

Next choose at least five other photos:

Tiny versions of these are used to build up a montage with the program choosing those with suitable matching colours for each position in the photo.
The end result being:

Some adjustment can be made to the number of columns of tiny pictures and their size.
Final output:

I'm sure they wont all be as easy to recognise as this one!