Before you spend a lot of time getting a grasp on the "DDS" files issue you must first select an image that is suitable to act as a "tile". See the attachment.
A suitable image is one that when placed adjacent to a duplicate of it's self, in all possible combinations will show NO edge between the tiles.
When this type of image is found it then needs to be sized the same as the "Path image" we want to replace. If the current image to be replaced in the terrain bank file is 1024x1024 then the new tile image should be the same 1024x1024.
The rest of the procedure is simple if the software being used is capable of saving a "jpeg" file out as a "DDS" file
I can't help you much with the procedure because I don't use "Gimp". I use a free copy of "Paint.net" (a very handy program for creating most dds files for Game Guru.
Be aware you only need a "Path_D.dds" file and a "Path_N.dds" file.
If you choose to try Pain.net I might be able to show you how change the "Path files" required.
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