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I haven't read this guide myself, so I might have referenced it without fully understanding it. Here's the method I use:
1. In the map's properties tree, find "thumbnailimages" and add elements equal to the number of images.
2. Create a new object in each element by clicking on the row next to the element and selecting "New," then assign a name.
3. Save and close the editor.
4. Open the map file with an archiver and place the DDS images in maps/SingleMissions/map_name/.
5. Open the XDB files that we named in step 2. In "destname," specify the link to the DDS file: , and also record the image size in the corresponding tags.
6. Save and close.
7. If the images still don't appear after all these operations, open the map in the editor, save, and close it.
There are no specific requirements for the DDS image subformat or size.