By pure coincident, my friend notified me that I could resize images with their proper ratio retained by holding down the Shift-key while dragging the image corner. This also made the printed version resize the images correctly, while retaining a good resolution. For me, this solved the problem :)

@Mac Dude 

Thanks a lot for your input. Making rotorshift an argument would be a lot better than defining it each time. I guess continuously defining the function somehow causes it not to upgrade any more after a certain amount of iterations. 

I ended up using your suggestions, but I also ditched the int() and used a numerical integration method by using the add() function. This worked. The int() function seems to be handling functions slightly different than the add().