I am correct to assume that I'm not allowed to use swears in graphics/pictures?

Yep. :) Pictures can't be filtered, only words can.

Would it be acceptable if I censor them like 'f***'? Or is that still a bit obvious...

...I'd assume as long as you gave a warning with the picture only being linked to, it'd be okay, but I'm not entirely sure.

Ohh, well it's not going to be an image that's being linked to, so I want to make sure. ^^