Images not visible in Excel Online

You could change the setting on the Document Library to open the files in the client application instead of opening via the web app. Go to Library Settings>Advanced Settings>Change the 'Opening Documents in the Browser' to "Open in the client application".

 

Or please see this link:

https://support.office.com/en-us/article/Edit-a-workbook-that-contains-features-unsupported-by-Excel-Online-7220105d-97ad-4c35-95c8-58d4bbfd14af?CTT=1&CorrelationId=f1934564-fc7f-4604-9b79-e075a89a9e7f&ui=en-US&rs=en-US&ad=US