Target iFrame

First of all, apologies, I'm sure this has been answered before, but I couldn't find anything after much Googling.

I have a large iframe with a placeholder image. Wix says that it's id is "html1". Below are a row of thumbnail graphics in a gallery. I would like for when a user clicks a thumbnail to have the large image, a pdf on another server, to load into the iframe above. So I place an address on the first thumbnail:

http://somedomain.com/portfolio/page1.html target="html1"

Wix completely ignores the target and tries to load the page into another window, but even that fails because it includes "%20target=" afterwards.

Does this need to be a javascript? It seems complicated for what should work with a simple html link.

Your thanks in advance.