The Excel-worksheet probably contains an embedded OLE object. Within Excel you can check this by right clicking each "image". When the context-menu's last option is
Format object, you found it.
To fix it, choose the option
Save as image and choose a location and name. Then delete the object, and insert the previously saved image.
There is also another way to check for embedded objects. An
.xlsx file is actually also a
.zip file. You can rename the extension so you can unzip it (or use
unzip filename.xlsx -d directory_to_unzip_to on the command line).
When the newly created folder contains a folder
xl/embeddings, and this folder contains at least one file, the excel contains an embedded object.