Click on the image in question so that the eight little squares show up around it. Then go to “Item > Extended…” in the Freeway application menu. Make sure is highlighted in the Extended Attributes box that appears and then click on “New”.
In the “name” box enter “title” (without the quotes). In the “value” box enter the text as you would like it to appear when the mouse hovers over the image in question.
Once you have done this click on OK to dismiss this box and then OK to dismiss the Extended Attributes box. Now when you preview the page your text should appear as you wanted.