Harvey,
you can edit your "editor_insert_image_button" macro. But first you have to create the "uploaders" group with the users that can upload images. You can give another name if you want.
Then, your macro should be like this:
<override_macro name="editor_insert_image_button">
<n.if.visitor.is_in_group group="uploaders">
<then.overridden/>
</n.if.visitor.is_in_group>
</override_macro>