Since you are in MOSS environment I assume that you are using the RadEditor for MOSS, which is control especially created for MOSS.
Regarding your CSS problem you can find information in the following help article http://www.telerik.com/help/aspnet-ajax/defining-custom-css-styles.html
Regarding the Image Manager, In the following article you can find information on it http://www.telerik.com/help/aspnet-ajax/image_manager.html . There is described how to configure ImagesPaths and ImagesFilters properties of the RadEditor.
is some additional information on how the ImagesPaths collection works.
By default, all the libraries from the current site are included in the
When you set path starting
without slash (e.g. "ImagesLibrary"), you will refer a local library -
library from the current Web. If you set path staring with slash (e.g.
/ImagesLibrary), you will refer a library from the SiteCollection
Lets open the SubSite1 ConfigFile.xml and set the following images path:
The ImageManager will load the SiteCollection/SubSite1/ImagesLibrary.
If you set the path with slash the ImageManager will open the SiteCollection/ImagesLibrary e.g.
Also you should know that if you set e library such as SiteCollection/SubSite1/ImagesLibrary, the currently logged user should have at least reading rights for all site collections of the path. Otherwise the ImagesLibrary will not appear in the ImageMangager dialogue.
I hope this helps.
the Telerik team