there's a good chance lack of light is your problem, but you're not giving much to work with.
This site is a good source for how to incorporate html tags into posts. the tag you're wanting is the 'img' tag and requires you have your picture hosted on a server somewhere.

Thanks grizzman for the info about images.
Here r my pictures...

http://i280.photobucket.com/albums/kk179/Danh_Vu/hydro002.jpg

http://i280.photobucket.com/albums/kk179/Danh_Vu/hydro003.jpg

http://i280.photobucket.com/albums/kk179/Danh_Vu/hydro004.jpg

http://i280.photobucket.com/albums/kk179/Danh_Vu/hydro005.jpg

So instead of the light, what else can be the problem?

when you preview your message, the image should appear as opposed to the http link. if it doesn't, something is wrong with your code.
the code should look like this:
<img scr="http://www.i280.photobucket.com/albums/kk179/Danh_Vu/hydro002.jpg"; />

Hmmm??? my last post didn't go.
I'll try again.
From a few of your pictures, the leaves looked limp. This is commonly a problem from lack of water, too much heat, or too high humidity. normally, the humidity problem is associated with too much heat, so it is not likely the culprit.
also, your perlite looks very dry. do the roots of the lettuce extend out of the perlite and into the nft runway? are your wick strings still present? does the bottom of the net pot sit directly on the nft runway? All these questions are getting at whether the plants are getting sufficient moisture.
A plant that young (two weeks or so) should not be having nutrient difficulties yet, thus water and light are the most likely culprits, though heat could be a factor too.

My roots extend to the nutes. I used a thin layer of perlite just to hold to plants in place. Two of them are okay. The other two are wilting and yellowing. I think it is the heat. My mom usually keeps the temp at 80 degrees Farenheit. Thats to high. What should I do when it is wilting?

From your pictures, I will guess that you are trying to intuit your way through this.....it's not working.

Get a book on hydroponics and google hydroponics on the web.

Here is a link that might be useful: Hydro Book