Jochen I'm not too different from you.  I have no art training at all.  I don't even have a lot of technical experience.  You're still a human being and your reaction to an image is still of value.

Getting response in a commercial sense is a completely different problem - I think great art can overlap with commercialism, but often it can conflict very strongly.  I'd suggest you look to the other vendors for advice there, examine their promo art and page layout, and look at their progress and evolution - some have been hosted here for a very long time, and their style and technique has visibly changed a great deal through different market offerings. 

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