I already wrote an article for my page about this, so I'll summarize:
Cipsoft hires an artist named Penciljack to lower costs and time.
Then fired their freelancers, who was the pinnacle from Tibia graphics quality (Óin, Izuael, Lord Ariakas, Mantus, Gyphonart and others)
- Jan (Penciljack) and Lyxoph never learned to sprite properly.
- Jan was only a concept artist and didn't know the patterns(or ignored them completely).
So every new update we see sprites that had appearance of the first scribbles of a child in paint, which have thickness of a toothpick and ignores AA.
- Most of that sprite are drawn in 1px thickness.
- And if you want a basic tip to know when a sprite is anti-aliased or not, is just check their border pixels: if they have light pixels, It's wrong!
As
@Arashel said, they look "shitty-blurry-sprites" ingame
About the legacy of the pinnaccle artists, a small piece of those arts was in the dead WOT Forum.
Me and some friends had access to the "small piece" (like tutorials, remakes and good sprites that weren't added in Tibia)
The other great part are in Phobos possession, although the game isn't updated with the latest sprites and probably will never be launched.
So if anyone of the above would be intrested PM me! 
