For beginners, Double Integrals can be difficult to visualize as the resulting diagram is in 3 dimensions. Here are some resources that can help:
1) YouTube Animation
This excellent animation explains the concept behind double integration and also polar integrals.
2) Wolfram Alpha Command: 3dplot
This command can help to visualize the surface . For example, in Tutorial 7 Q3, what does the surface
looks like?
You can type “3dplot z=x” into WolframAlpha: http://www.wolframalpha.com/input/?i=3dplot+z%3Dx
Result:

This makes sense, since means
, i.e. we can see that the surface should be a plane.