A picture may be helpful, but not always. I've seen a number of pictures of electronics showing the face plate only, recessed in an equipment rack and taken in dim light. Pictures of the back of electronics can be more helpful, if they verify the seller's description of the model or upgrade version and the country for which the unit was produced.
I agree with Rrog that pictures aren't very helpful in disclosing small blemishes. I'd rather see a detailed list of nicks, scratches, rubbed lettering and other blems.
More important than what they allow you to see, good pictures indicate a level of seller conscientiousness and care that are reassuring.