Its difficult to comment on the legality of using an iframe.
An Iframe is only a nested browsing context and so technically you can use it create a mashup provided you have control over what web content you are loading into your iframe. Hence it is a potential security risk for those who implement iframes and are not technically savvy enough to prevent malicious code from being injected into it , thus making your web visitors unwitting victims.
Come up with a list of sites you would like to link to , and analyze each sites T&C of usage, that should help.
I did have this same question as you regarding Pinterest when i created my first board for a game art project i was collaborating on with an artist friend, as we began to pin images from all over the net.
I decided to keep the board private and use the images only as inspiration, not commercially in any way.