Paralax broken updating from 2.3 to 2.4.1

I have just updated my theme within wordpress from 2.3 to the latest 2.4.1.

I have a one page site i am working on with multiple paralax images in. Since updating two of them seem to have broken. On scrolling into view there is no image displayed just the background colour, as you continue to scroll the image begins to come into view from the top. This seems to get worse the further you go scroll down the page.

I have attached an example of what I mean.