You can rotate images using the Transform Shader, but I agree that we should put rotation controls on the Image Map Shader.
To use the Transform Shader, connect the Image Map Shader the Transform Shader's 'shader' input, not its first input. It will be easiest if you use 0,0,0 for the position in the Image Map Shader, then reposition it using the 'translate' parameter in the Transform Shader. This is so that rotation can occur before positioning, as the other way around is quite a headache.