Here: http://endlessparadigm.com/forum/showthread.php?tid=231

It's quite complex, so to summarize it, you'll need to start at 0x11E.  The value there should be 0x01 - change it to 0x00.  Now add 0x38 to 0x11E, go to that offset, change the value from 0x01 to 0x00.  Continue by adding 0x38 again to the last offset and changing the value.  Keep doing this until you've done it for all the images (eg, if there's 10 images in the RCO, you'll have to do the above procedure 10 times).

It's lengthy, yes, but bstronga is known to not mind doing repetitive tasks :P
I'll see if I can hack up an RCO Editor which won't automatically compress things.