You can find some:
http://www.facebook.com/tibia (newsticker official news about it below)
"Looking for a new desktop background? We uploaded a new wallpaper on our official Tibia Facebook page http://www.facebook.com/tibia. It is available for different resolutions. Enjoy! "/https://www.tibia.com/news/?subtopic=newsarchive&id=2037
You can also find some details below:
https://www.tibiaqa.com/12327/how-has-the-client-background-changed-in-years?show=12327#q12327
https://tibia.fandom.com/wiki/Background_Artworks
Unfortunately, there is no official document made by Cipsoft, which contains all of their header artworks or client background, but you can find quite a lot of them in Cipsoft's FANKIT:
https://www.tibia.com/forum/?action=announcement&announcementid=87&boardid=89516
Another source of the official artworks (not only header or client backgrounds but also the smaller ones like events artworks, outfits, or mounts) may be TibiaFanart:
https://www.tibiafanart.com/official-graphics/
You can also find some of these illustrations in the art corner on Tibiopedia.pl:
https://tibiopedia.pl/gallery/artcorner/page/1
Loqhakk - Applying to TibiaQA team as ModeratorJuly 26, 2024
Nareull - Applying to TibiaQA team as ModeratorJuly 14, 2024
Looking for new TibiaQA staff membersJuly 12, 2024