Thumbnail generation in ListView and download from server

Not involving rotation but when I scrolled very fast:
it first tried to display img1-5 on screen1, after scrolling it should display img30-35, but instead it first showed the thumbnails for img-3-5 (as img1-2 already have been displayed on screen1), then the new task showed the real thumbnails for img30-35.
As I have read the problem is that the rows in the list are not destroyed but re-used and this leads to old references if you scroll fast and use a aSyncTask to get the images.