From The Document Foundation Wiki
- click on your image for the image toolbar to appear ( ▸ ▸ );
- click the Rotate icon on the toolbar (a semi-circle arrow). Note that the green handles around the image turn red;
- move your mouse over one of the red handels around the image (at one of the corners of the image). Note that your mouse turns into a round, two-headed arrow; and
- once your mouse turned into the arrow, left-click, move your mouse up or down, and release the left-click.