If you want to rotate a Partial Image around a specific point, you can use the Pin Tool to mark the position on the page.

1. Select the Origin Tool in the Tools Palette, or press Z on your keyboard to automatically select the tool.
2. Click on the Partial Image you want to rotate. This activates the Partial Image. You will notice nodes appear around it. 
3. Select the Pin Tool from the Tools Palette, or press N on your keyboard to automatically select the tool.
4. Click in the image where you want to rotation axis to be. This will add a pin at that point:
   
   
5. Select the Free Rotate Tool from the Tools Palette, or press R on your keyboard to automatically select the tool. This puts a square around the image:
   
   
6. Drag the corners of the box to rotate the to Partial Image. 
   
   

If you open the Info Window (Window Menu > Info), the angle will be displaying as you rotate: