Note: This can be accomplished without using the terminal, but I'll use it here because it takes less time
Note 2: This guide asumes you have TF2 installed in the default directory (/home/user/.steam/)
Note 3: TF2 should be closed during the creation of the first decal image, you can keep it open the next time you create one (This can lead to permanent visual glitches)

Step 1: Open a terminal of your choice

Step 2: Create the folders you need by doing:


Step 3: Close the terminal, your work here is done.

Note: I'm using GIMP here because it is the image editing software I like. You can use any program that can export to PNG
Note 2: Because I'm big brain I accidentally saved the file in an incorrectly named folder "Script". Please ignore it and follow Step 6.b

Step 1: Open GIMP.
Step 2: Create a new file or CTRL + N
Step 3: Make sure that the image is EXACTLY 128x128 pixels
Step 4: Put whatever you like in it, copy and paste an image, draw something, this part is all yours. For this, I'm using a red crewmate from Among Us
Step 5: Export the image or CTRL + (ALT +) E
Step 6: In the dialog box, go to your home folder (Or where the Steam Library is), type ".steam" in the name bar and press enter, you should be dropped into a hidden folder.
Step 6.a: Navigate from that folder to the folder named steam > steamapps > common > Team Fortress 2 > tf > custom > userstuff > scripts > items > custom_texture_blend_layers
Step 6.b: Type "paper_overlay.png" in the name bar.
Step 6.c: Click on export

Step 1: Launch TF2
Step 2: Go to your backpack, and use the decal with the Objector you want to paint
Step 3: Chose the first option (It won't use your Profile Picture)
Step 4: You should see the beautiful picture you chose in the Objector now, confirm that you wanna paint it.

And you're done!

Now you can repeat step 2 without reopening the game, it will automatically update every time you use a decal.