How do I skew an image in Keynote?

If by "skew" you mean rotate a graphic object, a 2D operation, Command-clicking any handle should do what you want. Screenshot:

Using the corner handle (pictured) will have a different effect than using a side handle.

Alternatively:

Rotate a shape, a text box, or an image

1. Select a shape, text box, or image.
2. Click the Arrange tab at the top of the sidebar on the right.If you don’t see a sidebar, or the sidebar doesn’t have an Arrange tab, click in the toolbar.
3. Drag the Rotate wheel or enter a degree value in the adjacent field to specify the angle at which you want to rotate the object.

On the other hand, if you wish to rotate a graphic object on its Z axis, a 3D operation, I think that you'll need to use a more robust graphics environment.

If you want that skewing to be dynamically visible to your audience, a combination of actions such as rotate and scale might be what you want.