Actually, the screens animation is part of the model, if you spawn one of those screens you will see that it already has the animations in it, its not a picture. (At least I am super sure it isn't.)
In Hammer, when placing the screen as a prop, you can select one of several skins. Each skin contains a different screen animation. If you want the screens, go into Portal's .GCF, find the .VTF's, and convert them to another image format.
Indeed, might need to outcrop the image, since it's part of the model skin, and jumbled with other textures as well.
Any idea where I might find these:?: