[Zope] syntax of ExtImage

[Mike Bunyan]

I have been unable to follow the syntax of ExtImage class
When called in the same folder in which it exists
<dtml-var mgscrdrbit.png>
returns
<ExtImage instance at 019D8AD0>
but I am unable to get img html string result in the form:
<img src=3D"http://localhost:8080/04_es/mgscrdrbit.png" border=3D"0"=20
alt=3D"descrip" height=3D"99" width=3D"100">
by using something like
<img src=3D<dtml-var mgscrdrbit.png>>

in addition, if called from lower down the tree, giving the absolute=20
pathname
<dtml-var /04_es/mgscrdrbit.png>
or
<dtml-var '/97_swi/bubardot.gif'>
produces
Error Type: KeyError
Error Value: '/97_swi/bubardot.gif'
and
This resource may be trying to reference a nonexistent object or=20
variable
 '/97_swi/bubardot.gif'.=20

on the other hand
<dtml-in "PARENTS[0].objectValues(['ExtImage'])">
produces all the information

Grateful for any guidance as the help file covers alll the=20
arguements, but no obvious (to me) dtml syntax.