Actively replace an image or phrase

I want a webpage to be load inside an full screen iFrame and a JavaScript to automatically replace images on the displayed site. Here is a more detailed description: Lets use [login to view URL] as an example (Swedish news site). In the top left corner there is an image of the letters DN.se. I have no access to their webspace what so ever but on my own webspace I put an html file with and iFrame with [login to view URL] inside the iFrame. When the site is loading inside the iFrame, I want a local JavaScript (or what language that is best for this) to replace the logo with another image. Both the IFrame html file and the JavaScript is hosted on my site. Nothing is changed on dn.se. I Also want to be able to replace text with certain phrases. If the site [login to view URL] contains a sentence like "Run to the hills", the JavaScript automatically replaces that with something else, like "Run for your life". Some css font and stuff, just the text. All of this has to load -before- the webpage is shown inside the iFrame. I have no clue if this is possible but I can't imagine it will take that much coding. (I know some myself but not near as good to make this possible).