Dropdown for "size for inline images" only shows thumbnail

hi,

I have three small drupal 6 installs, all using image/img_assist. While two work as intended, on the third I have a slight issue with img_assist.

In the Popup to choose and insert images, in the dropdown for the inline image size, I only get the "thumbnail" option. Whereas in the two other installs I get all defined sizes to choose from.

I compared settings between the different sites but cannot figure out the difference. What do I have to change to allow all defined sizes for inline images?

thank you