With those cameras is better to capture at high resolutions and resample down to elimitate noise.
A good camera should do this internally but those (Axis 205) cheap ones captures at the choosed resolution directly.
I like to capture at 640x480 to have a better image but also like to process and store at 320x240 to have less CPU and disk load, and to have better multi monitor window sizes.
If the captured resolution is not the "Expected size" zoneminder (Line 2487 zm_monitor.cpp) sends a warning. I think it should just resample, using netpbm if avaiable.
UPDATE
I've found a way to do this using the internal scale function in zm_image.cpp
Here's the patch
