There are several ways to achieve the TV scanline effect. The most straightforward way is to alternate two lines, each 1 pixel high, of similar but slightly varying color values (for example, ff6666 and ff3333). Then just place this alternating two-line pattern in the background of your design as desired. To fill a large area with a small repeating pattern, first select the pattern you want to use and choose Edit, Define Pattern. Next, select the area you want to fill, and choose Edit, Fill. In the dialog box that appears, choose Pattern from the pull-down menu and click OK.
Another technique is to create your scanline pattern using the method just described, save it to its own Photoshop layer, and then place that scanline layer above the other layers in your collage that you want to affect. Next, simply decrease the opacity of the scanline layer until grooviness occurs. Flatten your Photoshop image, slice it into manageable pieces, and save those pieces as gifs or jpegs to be called into your web page.
A third, more drastic technique (for Mac users only) is to use Deep Devices' "RetroScan" Photoshop filter (freely downloadable from http://shareware.cnet.com/shareware/1,10269,0-16170-501-0-1-3,00.html?qt=retroscan&ca=16170). According to its creators, "RetroScan is a professional quality Photoshop Plug-In that simulates the effect of rescanning a noisy TV signal. It provides an easy way to achieve the fashionable 'cyber punk' effect for print, multimedia, and video." I couldn't have said it better myself. It really is a groovy piece of software, allowing you to customize scanline size, scanline intensity, image ghosting, and even the amount of TV "snow." Use it sparingly. A little goes a long way.