First, you must go to Progam Files (x86) in that folder, find your Steam file. Click on the steam file and go down to steamapps,common,GarrysMod,garrysmod,dupes.

Once you find your dupes, see all that gibberish junk? There should be one file next to each dupe thumbnail, right click on the dupe thumbnail of your choice. Go down to the third option (edit) and click edit. Then it might take you to MS paint which is what i use to edit them, now if you already have a premade picture you can use that, or just take one off of google. Then all you have to do is just resize it if you need to, then fit infront of the checkered junk, you can edit it there if you need to. Then Exit and save the file. I reccomend not editing it with gmod open, as changes might not take place to your dupes immediantly. Then reopen gmod and see if it worked.

If you read all the way thru this guide good for you. its not very long so it shouln't be a problem, comment if it worked and if it did, leave a like! I was taught how to do this by Eaglezero6205 so thank him for most of this!