You site dosen't have a problem with alt text apart from the fact that you are using it innapropriately. What I think that you are looking for is the "tool-tip" that IE displays when you a hover an image with alt text? If so, IE wrongly uses the alt text and Firefox does it correctly I think, with the title attribute. It's not your site that can or can't display the alt text, it's the browser.
As for using alt text to say "click to enlarge" that's not such a good idea. The use of alt text is to provide an alternative text for the image in case a viewer doesn't have the images displayed or is using a screen reader. It should be used to describe the image.
If you want text to display when someone hovers over an image, use the title attributes. The other answer was right, you should use the alt tag for text that will describe your image to someone using a screen reader. Make the "click to view a larger images" a separate text link beneath the image, or something general at the top of the page to click on the image to view a larger image.
See the tips on the w3c image tag page. The w3c site is a great resource when you're scratching your head and trying to do things right: http://www.w3schools.com/tags/tag_img.asp
I just went through this. We ended up putting helpful text in both the alt and title tags so everyone on all browsers would see something when they do a mouse over.
set img label ,such as
<img src="yourpic.jpg" alt="yourtext" />
thanks for your help guys. title was what i needed