[jcl]

True, ownership of the preliminary work can be faked. However, without intimate knowledge of file formats and graphics algorithms it would be challenging to, for example, come up with an Illustrator vector file that rasterizes byte-for-byte to the scaled-down logo images copied from his portfolio. (assuming the thieves copied the graphics directly and didn't think to make their own vector file first...)

It may even be enough for him to have a credible high-res, layer-separated Photoshop file; someone who does not have the skills to produce their own original artwork probably also lacks the skills to convincingly fake a high-res version of someone else's artwork.

(Not sure if this has held in court, but I'd bet it has come up before.)