As most people don't have their own photos of celebrities, in order to 'Photoshop' famous people the designer would need to use somebody else's photos. This is obviously problematic with regard to copyright infringement. Had you considered this?

Thanks for contacting us TonyAlanCooper. We have considered this. You have the right to fair use of public images, as they are being used for parody. We also ask you provide links to source files you use. Please let us know if you have any further questions. Best wishes...