The file that you're downloading from Tableau Server isn't an Excel file, it's a .csv file. The problem is, .csv is a text based file format, while images are binary data and the two don't play nicely together. Unfortunately, you're not going to be able to export a .csv with an embedded image or any special formatting beyond new lines and commas.
There is this article from Tamas Foldi about a tool they have created to export data directly into Excel.
Haven't used it so cant vouch for it, but may be worth a try
All the best