I have this background image which must be slided in a way that it is repeated by y in order to follow the page's height.
The problem is I can't figure how...
Typical method is to slice off the top and bottom edge, and repeat the middle section in vertically.
For your case of a hand-drawn vertical edges, it's rather tricky as you need to slice it such that the middle section can be tiled vertically without looking disjointed. In other words, you need to find 2 horizontal slice lines which match top-down smoothly.