I can't reproduce the result that you have shown in your post, and it isn't alternate text.
However, see if this helps. Using Bridge, edit the IPTC metadata of the Description of the images used in the data merge to read "test".
Back in InDesign, go to the variable image field and click on it with the right mouse button to call up the contextual menu and select Object Export Options.
From here, go to the tagged PDF tab and change the dialog box to the following:
Export the interactive PDF as you have done, and run the action that you made that strips out links. What happens in this instance?
On my screen, the tooltip and the URL "duke it out" to see what should appear, UNTIL I run the action that strips out the links, then only the tooltip I made in the description panel of bridge appears.
