May I suggest that this be changed to maketx:read_local_MB and expressed in MB?  An 'int' may not hold a value large enough for some people's preferences.  Also I think we should clarify that 0 means "the system should make a good guess at a reasonable threshold", not necessarily that it will use ImageCache for even the tiniest of files.

Food for thought: This number specifies the size cutoff between allocating and reading the source image into the IB itself, versus relying on an ImageCache. (The former is faster, the latter allows you to maketx immense files while maintaining low memory footprint.)  Does "local" capture the idea in a way that's not confusing, or should it be called something like imagecache_size_threshold?  (Too wordy, but you see what I'm getting at.)