I think the most effective solution would be weighted keywords--each contributor gets a certain number of "points" per image, and assigns a certain number of those points to each keyword.  The more points, the earlier the image shows up in search results for that specific keyword.  A would-be spammer would be wasting his points on irrelevant or peripheral keywords and reducing the number of points he has to "spend" on other, more relevant keywords.  This solution would also improve the overall accuracy of search results.  Apparently this idea has been floated several times, but never caught on.