Just from my experience with toon rendering ... you don't need an image in IBL ... it just floods the whole scene with light. Adjust it to your scene to bring out the details and colors in a toon render. With toon rendering you need to look at lighting from a whole new perspective because you are trying to emphasize the colors and the line work ... which is why the tut refers to their lighting as superflat. The only time you want a lot of shadows is if you have noir scenes ... and again adjust your lighting accordingly.

Boni

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"Be Hero to Yourself" -- Peter Tork