Actually, the mouse over behavior is not a good indication of whether there is an alt attribute, as the browser decides what to display. You have to check the HTML. Actually, in this case, the image on the first page does have an alt attribute (see attachment), but the browser (Chrome in my case) may not display alt attributes but just title attributes. Search engines will find it though, if that's your concern. The image on the second page does not have an alt attribute, but it has a title attribute.
I'm not sure which widgets/module you're using to do this, but most probably you can customize the template to get what you want. I would recommend to either consult the documentation or submit a support ticket for that.