"Oft repeated, but incorrect. Look up the law in question, it says things like "that the defendant knowingly or deliberately did ...". Showing that there is reasonable doubt that you knowingly or deliberately obtained the nasty pictures, for example by demonstrating the presence of malware such as mentioned in the article, and the pictures only being in such locations as would be caused by such malware, would indeed constitute an effective defence."
You willing to TRY that? "Innocent until proven guilty" is just ink on a page to the plods and judges who feel the need to exert power over the plebs. Anyone who doesn't will likely find their career wrecked by planted kiddy pron.