I have a fairly large (entire bicep area) tattoo that I've had since I began bodybuilding.
I got it after I had begun T, however my arms were basically stick figure arms when I got it done, and have gained probably 5" since then.
It hasn't "stretched" the tattoo visibly, it still looks exactly the same as when I got it - it's just that the area where it's placed is now larger than it was before.
When I flex my arms, the tattoo does change shape - but again, not drastically. It's a clipper ship, and when I flex, it looks like the ship is "charging" so thats kinda neat.