Actually, it's pretty simple.

1) Get an image of the Tx you desire.
2) Convert the image to TGA format (using something like Photoshop)
3) Adjust the size of the image to 256x256 (be sure to do this or it will not work as a Tx Icon)
3) You may want to add an alpha layer that is a mask (if you know how to do that, but it's not necessary)
4) Save your file, giving it a name such as: myCoolTx.tga (be sure to use .tga file extension and do NOT use compression)
5) Copy your file to Graphics\RadioEquip\Controllers in the FS One installation folder.