Current questions and answers regarding the map editor.

First, you need to prepare the images (width 240, height 180 pixels) and save them in dds format (you don't need the DXT type, but 8888; the image editor should ask you this when saving). Then, go to MapPropretiesTree -> thumbnail images -> add, and in the row, click 'new' and edit the created file. It is best to make sure that all fields copy the corresponding fields from an existing image of this type (for example, maps/singlemissions/a2s1/thumb_sm1_1), but we won't need srcName. This completes the work in the editor. Now, close it and open the map archive. Go to maps/singlemissions/map name and find the xdb file that we recently created in the editor. Copy the image into our folder, and in the file, in the DestName line, in quotes, write the name of the image (the path is not needed). Archive it and enjoy.

Added 10 minutes later
You might also need to delete the SrcName line when editing the xdb file.