[may-bug]

I would, but still haven't found out how to do so. I've tried renaming them in the texture files of already existing maps, but that doesn't seem to work. Nor do I apparently know which texture files should be replaced in the first place.
Any clarifications on how this works?

You have to replace, in the load.ini file (with notepad), the name of the texture you want to change by the name of the new one. Example:

[FIELDS]
LowLand0 = land/winter/LowLand_1.tga
LowLand1 = land/winter/LowLand_2.tga
LowLand2 = land/winter/midland_2.tga
LowLand3 = land/winter/mount.tga

>>replaced texture names:

[FIELDS]
LowLand0 = land/winter/Mbug_sk_LowLand0_Hiver.tga
LowLand1 = land/winter/Mbug_sk_LowLand1_fields_Hiver.tga
LowLand2 = land/winter/Mbug_sk_LowLand2_fields_Hiver03.tga
LowLand3 = land/winter/Mbug_sk_LowLand3_fields_Hiver.tga

Make your tests in FMB.
That works with all maps, just check the name of the new texture is right, or the game won't load the map. If the game does not load the map, hit Shift+Tab keys. You'll see "... can't load xxx.tga" or something like this at the end of the list. Check for xxx.tga in your load.ini and correct it with the right name.
Hope it helps,

and many thanks at all for advices and kind words ,

may-bug.