Thanks for your reply.
Can you clarify what you meant in your second point? I would especially like to know the end of the sentence "There's only two..."

Actually drawing into a single image is not feasible, because the size of the image would be very big.

I guess the only option will be to listen to the scrollbar events and draw only what i need to display (as you mentionned in your points 2 and 4).
I will try to find some examples on the internet and hope it will not be too complex to implement.