I wan't to have a special theme for images/ gallery, and tried to use the "?wptheme=specialtheme"-switch for the gallery pages, but I noticed that the setting are stored in the cache.

There are three ways of sorting this out:

1. Remove the Cache part from the plugin code
2. Add something in the plugin code that will use a special theme on a special page
3. Make a plugin for this special assignment (I haven't found any yet...)

Or - do you have any suggestions?

As far as I know, WordPress doesn't have the functionality to assign specific themes to specific pages or sections. Perhaps a plugin could be written. More robust content management systems such as Joomla and Drupal have this feature.