How to use Clone Brush in Image Editor? I can use 3D cursor as reference point, when painting with the clone brush in the 3D viewport. But how can I use the clone brush in the image editor? 

I guess I will have to use the 2D cursor as the reference point. The problem is that the 2D cursor is available in the UV editor but not in the image editor (paint mode). 

Does anyone know a solution to this?
Clone Brush in the Image Editor (2D) doesn't work at all the same -you cannot use the cursor as a reference - you instead use a transfer or clone image that is the same size as the target image, and then move it about with rmb to position it and then paint through to clone. This is a VERY limited functionality, and there is no rotation of the image. For this reason, I prefer to use Stencil Brush Texture instead since we can rotate/scale/move and even set the UV to be upside down/repeating etc. Honestly, that is why it is sometimes better to just work on a 2d image plane in the 3d viewport than to waste time with the image editor for anything other than fast paint dabs or Fill tool control.