Are there any plans of improving the 'join images' tool

[gendalf]

it has trouble minimizing wasted space, or I'm doing something wrong.

[cday]

It would probably help Pierre if you could upload some test files [better in a ZIP or similar archive] and give the detailed steps to reproduce or settings screenshots, and if possible indicate your expected result...

[gendalf]

the idea is to either have it by default or as an option to selectively ignore the number of rows setting in favor of space efficiency in places.
expected result of the same thing as in the OP (each color = own picture):
1) 3 rows mosaic centered by the right side
2) 2 rows mosaic centered by the left side

[gendalf]

also add an option to include file names