Hey there! I'm gonna show you how to replace a dupe icon! I'll be using GIMP for this guide, but you can do it on any other software like Photoshop.

So let's start, shall we?

1. First, go to your Folders and locate 'C: > Program Files (x86) > Steam > steamapps > common > GarrysMod > garrysmod > dupes' or 'saves'. Then, right click the dupe/save image you want. Hover over 'Open with' and select GIMP.
2. After GIMP has opened up, click 'Files' and then 'Open as Layers...', then select a replacement dupe/save image.
3. If the image is too small, use the Scale and Move tools. For better results, you may want to use 'Filters > Enhance > Sharpen (Unsharp Mask)'.
4. (NOT REQUIRED) Delete the lowest layer.
5. Click 'Files' again and then click on 'Overwrite'.

It's the end of this short guide, but don't worry - I'll eventually make longer ones. I hope you have a great time duping!