Make a shader something like this:
textures/yourmap/yourmovingimage
{
surfaceparm whatever
{
map $lightmap
}
{
map textures/yourmap/imagename
blendfunc whatever
[/b]tcMod scroll xSpeed ySpeed[/b]
}
}
The italic stuff is all stuff you need to set depending on your needs. The bold line is what will make your shader scroll--it will move horizontally at speed xSpeed and vertically with speed ySpeed (I think those values are measured in textures/second, so a tcMod scroll 0 1 would make the image move upwards once every second). For more detail, check the q3a shader manual (
http://www.shaderlab.com/q3map2/shader_manual/).