Techy

image height = 5000;

var number of images = Math.round( image height / 2000)
// result = 3
so we will create 3 images of dianmentions
1) (0,2000) * (0,2000)
2)(2000,4000)*(2000,4000)
3)(4000,5000)*(4000,5000)

you can create above diamention images after croping the unwanted area. (see how to crop in anothe post)

and combine the same using some server side lanugauge.

as soon as i make a proto type i will put it on web for refrence but this is the idea how we can achive this.