Feature Request: Concurrent downloads / maxThreads setting

[torridgristle]

Currently the maxThreads is 4 and it will always download 4 images at a time. I would actually like to use a smaller number so that my internet connection is usable elsewhere. Seeing as how there's already a clamp on the number to keep it between 1 and 6, I reckon there might as well be a user setting for this value.