I think the OP may be using one of the photo pop-up objects within Web Designer, ie not a photo widget.

If so, open the Page & Layer Gallery and click the eye icon to make one of the pop-up layers visible, you'll then see the content of the pop-up layer and can edit the text.

Repeat this for the other pop-up layers you want to edit, then make sure you hide the pop-up layers and click on the MouseOff layer to make that th eactive layer again.