If you build your family tree online, please read the following article: How do I change the photo appearing in a person's card in my online tree?
Here are the steps to replace a Personal Photo (the thumbnail face shot that shows on the genealogy card in Tree View in Family Tree Builder) with a new one:
2. A new screen will appear showing all the photos associated with the individual.
3. Select the photo you would like to use and click ‘Edit’.
4. Next, find the ‘Associations’ tab. Here is where you define the personal photo for an individual. Click on the person’s name – this will activate the marking tool.
5. A personal photo will always have a blue box around the face when you are in the Edit Photo window. A photo which is only associated to a person, but is not the personal photo, will have a red box around it.
6. Click on person’s name next to ‘Associate’ button to activate ‘Personal photo’ option.
7. Check ‘Personal photo’ option, mark the face of the individual and click ‘OK’.
If you would like to use a picture that you haven’t added at all yet, once you go to the list of photos associated with this person, add a new photo, using instructions described in the following article: How do I add a personal photo in Family Tree Builder?