I haven't heard of "PIXEL LAB", although I have used Canva in the past for non-YouTube related projects. From what I've seen with regards to YouTube thumbnails, Canva is mediocre at best as it's mostly made up of pre-made templates.
Instead, I use possibly the closest thing to Photoshop CC/CS6 that is web-based - Photopea. It's a nice online version of Photoshop that gets the job done. I've actually switched from the actual Photoshop application to using Photopea. Might make a video on it in the near future.
I've also used Pixlr in the past, which is okay, but again, not as great at Photopea's editor.
Hope this helped, best of luck!