It's not exactly slang. More idiomatic if anything. But it is informal. The idea here is that someone is trying to make a connection between two ideas - and just like a physical connection sometimes it's difficult to actually connect the two things. So 'you are reaching' means that the speaker believes there is no real connection between the two ideas (an ancient picture that resembles someone as evidence of past lives). We have another common expression "That's a bit of a stretch." which expresses a similar idea.

In this context the expression means that someone is saying something that is far removed from reality.