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.

At that point, you can start FS One and edit your transmitter. From the Edit Transmitter menu, you should select the "Tx Icon" button near the top of the screen. This will allow you to select your new Tx Icon, which should now appear in the list of possible Tx Icons. Simply click on your selection (you should then see your Tx Icon that you created) and click Select. Next, click on "Ok" in the top right. There will be a pause as your new Transmitter data is written to disk. You will now see your new TX Icon for that radio.

Enjoy.