Due to Same Origin Policy restrictions, you can not access contents of iframe if it is running a page from another domain. There are solutions to same domain policy like
Check out Tomodo. It is just to give you a hint how they used proxy to bypass same origin policy constraint and access iframe content. So the implementation idea goes like this
Please note this is not a trivial task and you may have to handle a lot of cases at your proxy like handling relative URLs, cookie reading by iframe_url etc etc. So go for it only if you need it desperately.
If you want to download some images for a particular domain, just ask your server side code to it for you. Your backend code will fetch the html of page and use some HTML parser like
to parse img tags and extract the source and fetch the images and download them.
PS: Just for some good information, please read Ways to circumvent same origin policy