whenever i play guitar i have my thumb usually on the side of my neck facing upwards, just below the fretmarkers if you get me? can anyone explain the various positions and what they're best for ? pictures would be useful
Thumb position is almost 100% your own choice, so long as your hand is comfortable, relaxed, and can apply pressure to the frets.

That said, though, a rule of thumb (no pun intended) is to place it behind your middle finger, somewhere near the middle of the neck, when playing barres and other chords. Otherwise, most guitarists will give you a different answer.
Mine's all over the place, I find myself having it on the side to fret the low E for Hendrix songs and whatnot more often
Take the Kama Sutra. How many people died from the Kama Sutra, as opposed to the Bible? Who wins?-Frank Zappa