Pasted HTML and Image size

Hi folks,
I am filling a blog with a few hundred Posts at the Moment. Because the posts are similar (mainly images and links) I am generating the HTML with a script and pasting it manually into WP. I’m using the media manager for uploading my images. So far, so good.
However, since i’m using a modalbox,i need to turn the images into thumbnails. But when i try to change them to the the thumbnail size (i.e. relink to the automaticly generated thumbnail), the image itself changes to another image from an earlier Post (one of them i entered directly without pasting the HTML)

Screenshot:
http://schmidt-lohne.net/problem.jpg