Hi Jon,
Thanks for posting in the community. We are happy to help you.
According to your description, I have tested it on my side, and I can reproduce a similar situation. When I clicked the Browse button, there was no response, but I found a workaround to successfully insert the picture into the header. Please check if the following steps help or not.
- Please make sure the picture is a local file.
- Click Picture under the Header & Footer tab. The Insert Pictures window will appear.
- Type any word in Bing Image Search, and press enter. The window will display !!ERROR!! as shown in the below screenshot.
- Please click "Work offline", then File Explorer should pop on, and you should be able to insert the picture. After that, you can select one cell, and the picture will appear in the header.
Sincerely,
George | Microsoft Community Moderator