I am not certain what the status of uploaded pictures is.
From Facebook's own pages:
Do I retain the copyright and other legal rights to material I upload to Facebook?
Yes, you retain the copyright to your content. When you upload your content, you grant us a license to use and display that content. For more information please visit our Statement of Rights and Responsibilities, which contain information about intellectual property, as well as your privileges and responsibilities as a Facebook user.
Additionally you have control over who can view your photos and also which application have access to them.
This is the relevant part from the license you are signing up to:
For content that is covered by intellectual property rights, like photos and videos ("IP content"), you specifically give us the following permission, subject to your privacy and application settings: you grant us a non-exclusive, transferable, sub-licensable, royalty-free, worldwide license to use any IP content that you post on or in connection with Facebook ("IP License"). This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it.
Imagine however a situation where you upload an image to Facebook that you perhaps may sell at some stage. The client buying the image requires an exlcusive Worldwide license to use this image for a period of x years. They pay top dollar for this privelege but you have posted it to Facebook also granting them the ability to use the license, or even sub license it. They decide to use the image to promote Facebook and this coincides with you client's licensed use. You could find yourself in breach of contract/license with your client and sued.
Rich Clark: Note the excerpt above from Facebook's ToS:
This IP License ends when you delete your IP content or your account unless your content has been shared with others, and they have not deleted it.
When you delete your photos, FB loses the non-exclusive, royalty-free license to use them. The “unless…” part is tricky however, because if your friends have shared and not deleted the photos, FB still has the license to use your photos.
[EDIT:] Sorry, I don't think I really answered your question.
Fortunately, FB tells you which users have shared your photos. Might be as “simple” as sending them PMs requesting them to delete your photos. Complications arise, however, if their friends then re-shared.
You can also explain to your client FB's ToS in regard to content you share on FB. Then have their lawyers talk :) On a slightly more serious note, I think your client's lawyers will tell FB's that they have licensed your photos, and that that license overrides FB's.