To add an image selector field to your system configuration:

<image>
    <label>Image</label>
    <show_in_default>1</show_in_default>
    <show_in_website>1</show_in_website>
    <show_in_store>0</show_in_store>
    <sort_order>70</sort_order>
    <frontend_type>image</frontend_type>
    <backend_model>adminhtml/system_config_backend_image</backend_model>
    <upload_dir config="system/filesystem/media" scope_info="1">customer/responsible_contacts</upload_dir>
    <base_url type="media" scope_info="1">customer/responsible_contacts</base_url>
    <show_in_default>
        <enabled>1</enabled>
    </show_in_default>
</image>