Here is an example on falling back to the generic image if no ISBN fields
are defined
\define makeimg()
<$vars isbn={{{[{!!isbn}!is[blank]else{!!isbn13}]}}}>
<$image source={{{
[[http://covers.librarything.com/devkey/dev-key-goes-here/medium/isbn/]addsuffix<isbn>]
:filter[<isbn>!is[blank]] :else[[generic image URL]] }}}/>
</$vars>
\end
<<makeimg>>
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