Of course the image is mine, just as an image of the painting in a newspaper or brochure would be mine, it's a copy freely distributed by the copyright owner. I'm perfectly within my legal rights to do whatever I want with it, within the confines of the browser.
I'm not claiming i have the right to sell it or anything, but I absolutely do have the right to decide whether or not it shows up.
*if it was legally acquired. Remixing illegal stuff would still be possession of illegal stuff.
It's simply a matter of common sense, legal precedent and the way HTTP works.
To claim otherwise would mean companies have the right to force me not to alter the content of their responses in my browser, in order to guarantee their ads are viewed. If this were true, all browsers and many plugins would already be illegal, as would ad blockers, whose legality has been confirmed in multiple court cases.
Ethics and morality don't enter into the discussion. If I want to support a site by viewing their ads, I can. If I don't, I don't have to. The choice is mine to make. Sites depending on ad revenue, meanwhile, can try as hard as they want to convince me to choose to view their ads, but they have no legal right, nor technical ability to make me, as long as we're talking about content that can be filtered out of an HTTP response by a browser I control.
This is legally false. If a digital artist creates a painting and puts it up on their website, and you go view it, it does not become yours. The artist retains the copyright to the work, and not only is it illegal for you to then sell prints of it, but a violation of ethics, to boot.