OK, thanks for the information. That thread provides a useful summary of what an author can and cannot do with source material, particularly in regard to lifting copyrighted material straight from source, and creating derivative works in photoshop or similar from those and claiming (directly or through non-attribution) it as their own work. In those terms, it is clearly wrong, but many authors appear to be unaware that what they are doing is illegal.
However, even the DMCA needs a reason in takedown notices. Does TWiki have some sort of 'discuss' area where any relevant details can be kept, or can a slighly longer summary be used in future? (eg: Removed: illegal use of copyrighted images without permission or attribution)
Edit: clearer English and more exposition