This is a very good post. Always good to hear valuable input from buyers. As a side note, also be aware that the keywords you add to images influence not only buyers on Dreamstime, but how people might find (or might not find) your images in Google. If you load your image down with tons keywords that aren't highly relevant to the exact subject matter of the image, you can dilute the specific relevancy for your image and hinder it's visibility in search. think about what people are really looking for either on DT or Google and speak to that as best - and as accurately - as possible.
For instance, you "could" add these keywords to the soup image: round, white, orange, gray, liquid, metal. While these are certainly all found in the image, they are so broad that they don't speak to what the image really is: a bowl of pumpkin soup.
Obviously this isn't a black/white issue, there's a lot of "mushiness" to the concept of keywords - as Jppi_stu points about the "vegetarian" keyword - but it is something we should definitely pay attention to.