Simple question:

I've seen amps/cabs with a painting covering the entire grill, or sometimes just a simple stencil-type design (like a band logo or even just a phrase.) How do people do this? What type of paint (if even paint) should I use? Will it effect the sound? Ect.

There are multiple designs I had in mind ranging from a full fledged painting (my dog emulating Anubis with an Egyptian background) to a stencil of a mustache. I imagine the technique I use should reflect the intended design, so how will I need to do various designs and how will they effect the sound?
Fabric paint in a spray can. You can get it at most auto supplies....in Australia anyway.
Should I take the front panel out and take off the speakers? Or is that unnecessary?