When you're in the addons folder, create a folder and call it whatever you want. Enter that folder, and create a new one named materials, enter it, and I think you can guess what the rest is. Just create folders until your directory looks like addons/(your folder name)/materials/lambdaplayers/custom_profilepictures.
I'm not entirely sure what you meant in your question, but I hope this answers it.

@somefoolouthere witch folders do i create first in the addons folder?

Oh, I think I see what you're trying to do now. You're trying to add your own, right?
You'll need to make a new folder in the addons folder within your GMod directory and add images into materials/lambdaplayers/custom_profilepictures (relative to the newly created folder). Assuming that you did what I said in the previous comment successfully, you can use the extracted folder for reference if you don't fully understand.
The images can be PNG or JPG, and their resolution doesn't matter.

@somefoolouthere the profile picture i placed in there arent showing, do i have to make them pngs and have them at aspect ratios of 184 x 184?

Relative to your steamapps folder: workshop/content/4000/3348117011/gmpublisher.gma
Use gmad.exe in your GMod's bin folder or a third party program to extract the file.

where do i find the file for the profile pictures in this addon?