Using Office 365, I created a new page in Excel. On the "Page Layout" tab I clicked in the bottom right hand corner to open "Page Setup" then clicked on the Header/Footer tab. Next I selected "Custom Footer."
Having decided to insert my logo in the Center Section, I next clicked on the icon on the very far right labeled "Insert Picture." A menu that allows one to either browse their own computer or browse Bing appears. My logo is on my computer so I will choose that option. Once I've made my selection, this appears in the footer "&[Picture]" without the quotes. If you look at the same row of icons mentioned above, you'll see that now the icon on the furthest right is available. It is labeled "Format Picture. Click it so you can shrink your logo to fit in the Footer. I usually adjust my pictures/logos by changing its height since I always know how much room I've created for my headers and footers.
Hope you find this helpful.
