new phones full of rubbish!

Its more than likely bloatware put on phone by Samsung or your carrier

 

Samsung make the Galaxy Nexus, however Google and the carrier are responsible for the junk on the the phone, it's probably down to the carrier.

 

The OP said advertisments for cornflakes and topgear, that is neither carrier nor manufacturer.
That is most likely from web browsing cache or an app.
As to why you can't see them on the phone, they are probably in a hidden folder, a folder with a . in front.
I had tv.com installed, it would keep a picture of every TV show you searched.
Blockbuster did the same thing for every movie.

 

I was looking at clock skins for Beautiful Widgets and ended up with hundreds of pictures of clocks in my gallery in a folder called "cache".