A lot of people keep asking that. It's from the original artwork print. The border is there to make Spider-man look like he is coming out of the frame the same way they do in comic books.
Personally, I like it. It's the same image used on the US 4K steelbook.
Picked up the French 3D edition of the first release for 12 quid and popped in the 4K disc that came with my Panasonic UHD Blu-ray player. Looks much better than this one TBH.