I have scanned my signature and can insert it just fine, but at the wrong angle!
How can I rotate it (90 deg)?
I've been having the same problem, but I solved it by opening the pdf file in Preview, hitting
File > Duplicate, then adding the signature in the newly created file. This corrects the orientation of the file so that Preview can identify lines and you can just click on them with the signature tool (from the Annotate/Edit toolbar) and add your signature.
This thread addresses the problem best:
Basically the issue is with Acrobat formatting scanned PDFs in a way that makes Preview think it is rotated.
The answer above about exporting to a new PDF and then trying to insert a scanned signature in the new PDF via Preview's annotation tools works to fix the problem.
To rotate the whole image:
To rotate only part of the image
You can also rotate signatures and other annotations, like text boxes or rectangles, by arbitrary angles:
Hold the Command ⌘ key and do a rotation gesture with two fingers on a trackpad.
As far as I know this only works with trackpads and is undocumented. It works on macOS High Sierra.
This is from a related question on AskDifferent: How do you rotate inserts in Apple Mac OS preview 7?