Copyright, Licences, Public Domain
The short answer is yes.
By definition, an open textbook is a book for which an open-copyright licence has been applied. This means that anyone is allowed to use, keep, share, and change the information within that work, without asking the author’s permission, provided the author is given credit in an attribution statement.
If using a specific image within a textbook, check the caption of the image and any attribution statements at the bottom of the web page for additional licensing conditions that may apply. Images within a textbook are sometimes covered by a different Creative Commons licence than the textbook in which they are used.