Think of the low res screen windows uses until windows is ready, with all the graphics drivers and such, to display in the resolution you have your desktop set to. Road Runner loads in much the same way.
The purpose for the loader screen is to provide some user feedback while loading the other components of the software. It does not resize by general.ini and does not support PNG files because it would require some extra processing.
I understand your point about pngs. I would still like to see loader screen resizing accoring to the general.ini in a future release. Guess this wouldn't slow down loading RR. I understand that this would be low priority .