When I try to create a .pdf from my terminal I get the following error:
GC Warning: Out of Memory! Heap size: 2040 MiB. Returning NIL!

This is prob. due to large images (it works if I lower the image size) but how do I instead increate the "Heap size" making it possible for me to process the larger images?

How large are these images? Are they being loaded from local files, or over HTTP? And how much RAM do you have on your machine?