Just had a quick look around and as far as I can see they are sourcing their images from three named freebie/freemium sites, Iconfinder, Pixabay and StockUnlimited. We've had discussions here about StockUnlimited and Pixabay

Looks to me as if Youzign is going after Canva's approach but you buy one time vs. pay per design. Templates and easy creation of common online sizes & formats (Twitter covers, FB covers, etc.). The video trumpets that you don't need a designer and can do something easily yourself. From this review: "I like Canva a lot, and it�s worth checking it out if you�ve never used it. Basically, Canva is a VW Beetle, Youzign is a Porsche 911."

Here's a video comparison/review - google for more; apparently other people see the two as competitors

https://www.youtube.com/watch?v=NoxbNB8GovY