I am using a clone brush on a model using CTRL+Left Mouse Button to define origin of the clone brush. Why it doesn't work when I'm doing the same in UV/Image Editor?
I'll show you the method, which does not work exactly like the one you mentioned (Ctrl+Left Mouse Click) but may be useful for you.
In Uv/Image Editor change the Mode to Paint.
Place the cursor in the Uv/Image Editor window and press T. It will display the Tools panel on the left. Choose the clone brush and an image, which will be the source for cloning.
Place the image wherever you want holding the Right Mouse Click. Paint by pressing the Left Mouse Click.