There are now a large collection of thought icons available by hitting the down arrow next to the thought name on the Thought tab.

 

This will now bring up an icon Library window:

 

 

From this window you can copy or delete an existing thought icon. Clicking the camera button will allow you to capture a screenshot, clicking the folder button allows you to browse to select the icon, and the paste button will use the image file you copied to your clipboard.

 

You can also select categories from the dropdown menu to narrow the list of existing icons.