In the UK at least, the desk in the picture is in an area known as "in the eaves",
Also think about storage, as built-in space can often be included, especially in the eaves. (-- Home Owners Alliance website)
or "under the eaves":
A bed tucked behind a curtain under the eaves makes for a charming den. (-- House & Garden magazine, Dec 2012)
In my experience, it would be understood that "in the eaves" means an area with reduced ceiling height - no-one would expect to be able to stand upright there.
Historically, the eaves themselves are the external parts of a pitched roof which overhang the outside wall. By proximity, the name is now associated with the part of the inside roof space nearest to the eaves.
EDIT to clairify - this only applies to the part of the space next to the (low) wall. Your question asks about the ceiling and could refer to anywhere in that space. I'd just call that a "sloping ceiling".