If you know what the image of the tv screen looks like then all you have to do is find the image in the my_objects folder and edit it. Since you want different images though you will need to make a new material file. Copy and paste the original one and open it up with Notepad (right-click, Open with, Installed Programs, Notepad).
material TelevisionMat
{
transparency_casts_shadows on
receive_shadows on
technique
{
pass
{
diffuse 0.05 0.05 0.05 1
cull_hardware none
cull_software none
alpha_rejection greater_equal 128
texture_unit
{
texture tvimage.png
}
}
}
}
The new image you made? Type it in here, including it's format.
To prevent texture problems, let's change the old material to a new one.