In order to unable Image upload tab
you need to set ImagesPaths and UploadImagesPaths properties. You can do this by modifying the respective ConfigFile.xml, which is located in Program Files\Common Files\Microsoft Shared\web server extensions\wpresources\RadEditorSharePoint\126.96.36.199__1f131a624888eeed\Resources folder.
In addition please consider upgrading RadEditor for MOSS to the last version, which now is 3.5.1 .
I hope this helps.
All the best,
the Telerik team
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