So if you followed the other skintexmod tutorials you should have landed here last.
Make a folder with the name of your skintexmod(as if you were going to release it)
Drag your config file and bodytex.png in to it. Drag that entire folder to skintexmod-Textures
Open up honeyselect, use your skintexmod on a char, find some nice pose and take a shot of the char using the skintexmod, this will be our preview image
When you are satisfied with how your preview looks save it as preview.png and next time you open up skintexmod it will popup as the preview image.
Heres what it should look like if you did it right