Just open an http connection request the whole page, then start requesting the elements in the page aka the webcam picture. Use a browser header from say Android running Chrome or an iPhone running Safari depending on the platform of the app. Hey you could just request everything on the page for good measure. Nothing requires you to actually display everything in the page and there is no way of them knowing whether you have or not
In the end if it displays in an ordinary web browser then there is NOTHING you can do to stop me scraping the image.
You're right that in the end you can get around any block, the question is how much effort you're prepared to put in to getting around the block. Some types of block can't be defeated using a stock web browser, you need extensions to do it. Photobucket started blocking embedding recently for users who don't pay a large fee to allow embedding in 3rd party websites, the only way to defeat the block is with a browser extension, which I'm told is quite slow.
My suspicion is that if Cairngorm were to try to block the webcam images, they will do so by checking the http referrer header. This header tells the server the address of the page where the image is to be embedded, if the referrer doesn't match up with the Cairngorm website, you could block the image. They could also start generating unique keys in the page, which have to be passed with the image request in the referrer. I could fake the referrer in the app and potentially retrieve any unique keys they might generate to detect faked referrers, the question is, it worth the effort and will there be a knock on. I'm not certain of the extent to which the owner of an image published on a website, (or in this case a webcam) has the right to demand that you take the embed down. If somebody puts in place a referrer based block, then it is clear that they want to stop embedding, so at that point you leave yourself open to a "cease and desist" request. That may or may not not have any legal force, but considering the current "bad blood" between WH and Cairngorm, I would have to discuss with Alan how to react in such circumstances regardless.
Hopefully this post won't put ideas into CG's heads...