Rotating a shape in OmniGraffle
 

Is there any way I can rotate a image at an angle?

Sure. Select the shape you want to rotate. Bring up the Properties: Geometry inspector. Use the little knob at the upper right to dial in the rotation, or type in a numeric value (in degrees).

You also mentioned rotating an image, which can be done the same way. Draw a shape, bring up the Style: Image inspector, select your image. If you don't want to see the shape you are using as a frame, click on the Style: Lines and Shapes inspector and uncheck the Stroke box.

For a quick rotation, you can also hold down the command key then click and drag on a corner/edge point.

Another tip:

Once you've clicked on the geometry rotation knob in the inspector or held the command key down in the drawing you can drag the cursor as far away as you''d like from the knob or shape. That can give you a larger radius of rotation, allowing more precise control of the rotation angle.

In fact, if you take some image rotation sdk, you can rotate your images in any angle.