How to concatenate URL string to get dDocName in the middle?

I have piece of code where a web rendition of an Image is shown.

<img src="<$js(#active.URL)$>" height="50">

But this will give the web rendition and I would like to have  Thumpnail rendition instead.

I know that I can get the Thumpnail rendition with the following url:

"<$HttpWebRoot$>idcplg?IdcService=GET_FILE&dDocName=UCM_00000001&RevisionSelectionMethod=LatestReleased&Rendition=Thumbnail&allowInterrupt=1&noSaveAs=1" height="50">

And the dDocName can be fetched from the active item like #active.dDocName or getValue("#active","dDocName")

Url is supposed to be inside "  " -marks.

How can I get the dDocName of active item included in string?

This may seam trivial, but I really am not used coding.