I have ~ 900 pictures (mostly photos) of varying sizes.

Now I want to create one big picture that includes all 900 pictures in a small thumbnail-like resolution. The tool should automatically resize the thumnails so that everything fits. All pictures should be added next to each other, so there should be no border/padding.

Each picture should be included exactly one time! (however, it would be okay to omit a few so that there is no empty space)

I'm looking for a tool (FLOSS, for GNU/Linux) that can do the job.

I tried Metapixel, but

• it needs an input image that should be "resembled" by the thumbnails, and
• it only uses a selection of all pictures.

I found montage, but couldn't get a result yet, because my system was unresponsible for hours after starting it. Maybe there are some tweaks?

AndreaMosaic is recommended in this answer, but it's not FLOSS (and needs Wine to run it on GNU/Linux).