We have great news about Excel for Mac! With the latest Insiders Slow update, you can now insert and format pictures in headers and footers. This feature has been requested quite a lot, so we're happy that it's here now. Just update to version 15.36 (170702) or greater and you'll be able to insert pictures in your headers & footers.
In the latest Insiders Fast update, just go to Print > Page Setup to set custom headers/footers, or go to View > Page Layout View and click in a header or footer field. Then look at the Header & Footer tab on the ribbon. There's a "Picture" button which lets you insert any picture from your computer or you can format a picture that's already been added to your header or footer.
Please try it, and let us know if you have any concerns, problems, or suggestions. Reply below to let us know what you think. If you have other requests, please visit http://Excel.uservoice.com and tell us what you want.
This is still just for Insiders Slow, but we hope to make it available for everyone soon.