Manually imported movable type assets to wp, now Edit Image doesn't work

Ok, I’ve spent a great deal of importing movable type assets to my new wordpress blog. This involved mainly to import the mt_assets table and manually insert the contents of it to the wp_posts table. Although a lot of work (of course I had to modify the paths), it is now basically working (all the movable type assets are in the media-library, the path is set correctly, the image shows up within the media library), BUT:

Once I want to edit an image and click the ‘Edit Image’ button (the one that takes you to the screen where you can rotate, scale, crop etc…), wordpress says:

Image data does not exist. Please re-upload the image

Also: if I want to include one of those images in my posts, I cannot set the thumbnail width or dimensions. The radio-buttons are just disabled..

I’m clueless why. Is it maybe because the mt imported assets are not in a <month>-<year> directory? (i’ve all uploaded them to a separate ‘mt’ directory within the files-directory.