I am happy somebody has already noticed it
In general, the favorites function is not that different from bookmarks in your browser. As you maybe know, the card generator stores all parameters in the URL. The favorites are these URLs stored locally in the browser (similar to Cookies). An advantage is duplicate checking for new favorites (it does store the exact same parameter combination only once) and the search function. The title is generated out of the template type, the price, the card types and the card title. But be careful, if you delete your browser data, all sorted entries are gone again.
Or if I program a bug...
Real favorites or exported pictures are saver.
So what to do with it?
I wouldn't use it to store entire expansion sets because there is no export function yet and as said, it might get lost easily. But for a quick access for current work-in-process cards or own templates, it might be just right.
Why did I do it?
I was in the mood for programming and liked to explore the possibilities of the browsers local store a little bit more.
Additionally, I did also add a simple web app service worker, that caches all card source files. Hopefully, this increases the performance of the generator.
PS: as already said by Freddy10, the image issue is most likely related to how that webpage serves the files. But I haven't found the reason...