Hi there. I moved to design since it seems your question is more image related than app related...
For the answer, I'm going to say that in general, any image included with iWeb, or a template, is not going to be high enough resolution for a print brochure. Even if you are desktop printing and didn't mind fuzzy/bitmappy results, the template author/photographer might not want you to use it that way (without paying more etc).
My suggestion is to find your own image (istockphoto.com etc), buy the high-res version, and create a low-res version for use on your website. You can look inside the theme folders for the image name and replace it.