You use a photo editing program, like Photoshop or Photoshop elements
John T's recommendation is the best, but you can do the Rotation in PrE. It is just not the best way to accomplish what you want. I use John T's method most often.
In PrE, Select the Still Image (Photo) on the Timeline (better than on Sceneline View Mode). Once it has been Selected, go to the Effects Tab, and then near the bottom, choose Edit Effects. When you click there, you will see the Fixed Effects (appear for all Clips), and you will want Rotation. There, just choose 90 degrees. Do not leave the Effects Control Panel, as depending on the Aspect Ratio of the Still Image, you might need to adjust the Scale (Motion>Scale), and possibly Position (Motion>Position) to get what you want.