Further to what @user568458 said in the comment above about my answer on the other question: Try writing it in your contract with the designer.
I AM NOT A LAWYER AND THIS IS NOT LEGAL WORDING. This is just a suggestion. You should run this by an actual lawyer, and I have no idea if this will hold up in court. But I think it's a decent start, and as a designer, I would have no problem agreeing to this.
"Client may ask Designer to provide artwork for Project.
- If Client requests photography, Designer will provide written proof
to Client that rights to the photographs were granted and/or
purchased from the owner of the copyright on the photograph(s).
- If Client requests artwork, Designer will provide written proof to
Client that rights to the artwork were granted and/or purchased
from the owner of the copyright on the artwork.
- If Designer takes the photo and/or creates the artwork, and Client
chooses to use the photo and/or artwork for Project, Designer agrees
to transfer the copyright of the photo and/or artwork to Client in
writing. OR Designer agrees to sell the usage rights for the photo
and/or artwork to Client for this Project.
- A separate contract will be drawn up between Designer and Client to
effect the copyright transfer/sale of usage rights for the photo
- If Designer cannot prove in writing that Designer and Client have
permission to the photo and/or artwork provided to Client for use in
Project, Client will refuse usage of the photo and/or artwork. In
such case, Client will not be held responsible for any plagiarism or
copyright violation charges against Designer. In such case, Designer
promises to say that the use of the photo and/or artwork was not
Client's fault or Client's request."