I realised that mine thumb isn't straight when I play. My thumb looks like Jacos, not that bad but still isn't straight. I play that way and I heard from someone that you should not do that, not break your thumb joint and keep thumb straight.

So can you post pics of your left hand position on back of the neck.

my thumb usually isnt like pressed right on the back of the neck, like i'm fretting with it or something, you know what I mean? Its usually like, sideways. Like the tip of my thumb is kinda pointing towards the headstock, but more towards me. Its not perfect but it works and it's comfortable.

People think to much when they play bass. Just go with feel.?

Your thumb position is completely unimportant. Everyone makes a fuss over it, but those same people don't pay enough attention to the rest of the hand-arm system (i.e. the part that actually does matter). Worrying about where your thumb is is like worrying about whether you put your arms in the right place when you walk. As long as you're not walking with your arms (or squeezing with your thumb) you're fine.

Thanks for your replies. I thought it was ok but it just bothered me and I had to ask. I know its stupid beginner question but I had to do it.