I just make those for you. Search "thomas custom". There seems to be a few variations of the Logo, so I just randomly pick one.
It is actually pretty easy to DIY. Here are the steps:
1) Download GIMP (free).
2) Google xxx icon png file (PNG makes transparent background, the easiest to work with. JPG will be more difficult as you need to take out the background and make it transparent).
3) Save the Source button file by viewing it in the search database, and CTRL-CLICK and pull it out to your desktop or folder.
4) Open up the Source button.
5) Open up the logo png file.
6) Under Image, scale the logo png to fit into the source button (the header should tell you the size of the source button, so you want to scale it to fit within it).
7) once done scaling, do a CTRL-C (copy) on the logo window.
8) Move to the source button screen, do a CTRL-V (paste).
9) Move it around if you don't like the default position. Usually, they should be in the middle.
10) Export it to a PNG file.
11) Import it into irule by "add image".