It's kind of hard to tell with the pics. The egg looks mostly black to me, I think there's too much light. At day 12 you should be able to see veins and the embry moving. Good luck!
Welsummer eggs can be SOOO hard to look into. To me it looks good. But with a welsummer egg what looks like an embryo developing could possibly be bacteria developing...
My EE's eggs were impossible to see through. All I ever managed was seeing the air cell and a hint of a shadow moving a time or two (and that was in a VERY dark room). I had 3 (out of 4) babies hatch out from them today.
All 4 had developed beautifully, I just had humidity related hatch issues or wouldn't have lost one. I am sure yours is good.