Okay, I've now discovered this treasure-trove:
http://taff04.wixsite.com/taffinexil...ighty-8th-mods
Here you can download a modding editor to open up and change/replace the various files in B-17 (you have to ensure that the game is patched to 1.0
before you load this editor into your B-17 root folder, or none of it will work)
Included in the OMFmod Manual are detailed instructions on how you can also work on these files yourself, using Photoshop etc.
One mod which caught my attention changes the terrain tiles to an autumnal colour; the bilious green of the landscape in B-17 is pretty awful in my eyes, so whether this will be an improvement remains to be seen. I'm going to look into the possibility of building my own snowy winter terrain for this... if I can change the look of it myself then that would be very exciting