1. In order to save a dupe that you have made, you need to use the duplicator tool in-game. Once you have saved it, it will appear in your dupes folder and the dupes section of the Q menu.

2. After this, go to C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\dupes on your computer to find every dupe you've made so far. Each dupe has two corresponding files ending with .jpg and .dupe consisting of varying letters and numbers.

3. To make a new thumbnail for your dupe, you must find the picture in the dupes folder you want to replace, cut and paste the name of the file onto the picture you made for it and put it in the dupes folder.

4. Next, you must load into gmod to see if the changes took effect. If it worked, you can upload your dupe to the workshop.