I would ensure the Tableau Server has permissions to access the external sources that are hosting the images.
There is a good blog on the Information Lab for using images in Tableau. https://www.theinformationlab.co.uk/2014/08/21/using-images-tableau/
If you download the images they also can be included with the publication of the workbook. https://kb.tableau.com/articles/issue/unable-to-display-images-in-published-view-or-dashboard
Hope this helps!