Display Photo from URL in iOS 8 Today View Widget
I was also looking for such a webcam widget and also couldn't find one to suit my needs.
But as you mentioned RSS there is one that can do that (Photo of the Day Widget by Francis Bonnin) however you can't set a refresh rate for it in the app/widget settings. I also couldn't get it to work with a webcam url (JPG url or page where the webcam e.g. JPG is shown) or create an RSS feed via existing online tools for the image link or webcam page. If you know how it can be done for webcams at other websites let me know. I'm trying to run a script on my own webcam with one or more webcams from other website by looking into this W3School RSS image element code. 

I did receive a good response from the developers Lennart Reiher (PhotoBar widget) and Francis Bonnin (which created e.g. Photo of the Day widget) which were thinking about creating a new app/widget after my e-mail to them. They said that the new widget could have one or multiple webcam URLs (e.g. to a JPG) and hopefully also have a refresh rate option in it. 

Also take a look at the huge iOS8 widget list (i'm also updating in the start topic almost daily at MacRumors forum at iOS8 sub category in the sticky widget megathread topic). I also created a Photo category there. 
If you found a widget or build one yourself let me know here and maybe also there.