Canola oil (also called rapeseed oil in many countries outside the US) is usually recommended. It’s what the cast iron manufacturer Lodge uses to season their skillets.
I've heard that a lot about flaxseed oil, it's really common. I've always used crisco, so I can't suggest anything else from personal experience. But this is a site I trust, and he has some alternatives on this page
I generally use Crisco, but that picture looks like maybe the oil was applied too thick? I get really good results with Avocado oil too. My process is to wipe the oil on with a clean cloth, and then try to wipe it completely clean. There will be a very thin layer still on the pan. I used to season in the oven at 500F, but I have lately had really good results on the range. I turn the burner up to high and let the pan heat until it is smoking, and let it smoke ~10 minutes. Once the pan is cool, I hit it with another thin coat of oil.
Maybe I did use too much oil initially, but I also think that since I kept using it while it was flaking it sort of got new layers of seasoning, but unevenly.