Implement More Efficient Group Photo Processor

[clif-os]

The current implementation is very slow (sorry lol). For 2000 images, processing can take more than 2 minutes.

We should look into adding a more efficient means of making the group photo.

A bonus would be completing this ticket as well in the process: #28

Maybe there is a way to achieve with the current image processing library (https://sharp.pixelplumbing.com/), but we should look into other libraries if needed.