In the situation where I see someone doing something and having fun I might say 楽しそう. For example, I might be standing next to a roller coaster looking at it thinking that this looks like fun. So I would say
楽しそう。乗ろう！ = This looks like fun, let's do this!
(Of course I don't know if this is a natural or correct thing to say in this case)
Now assume I am looking at someone's pictures, say, of this roller coaster. Say, they went to the fun fair today and came back and are now showing me pictures.
How can I say "It looks like you were having fun."?
Some ideas I have are
The problem I have is that it makes me think that the past tense verb だった refers to the 楽しそう so that it would mean
This looked like you are having fun.
When I really want to be saying
This looks like you were having fun.
So I guess I want to put 楽しそう into past tense but 楽しそかった sounds wrong, too.