I am sorry to write a new post here, as I have already found that others have had some similar problems with upgrading from a Drupal 6 page that uses image assist tags for showing images. However, I have not been able to solve the problem, I so would be very thankful if anyone could please explain how I should make this work.
The upside: I have managed to migrate everything important, so apart from none of the pictures shown with image assist on the old site, everything else works great.
I have tried a couple of the php scripts for updating the old database before updating, but I have not gotten it to work.
Is there any way to convert all the old image assist tags to html either before or after I have upgraded my site to Drupal 8?