I'm self studying a textbook and sometimes I do not understand the explanation the author is trying to give for a concept. Is it OK for me to write out what the textbook says verbatim and/or take an actual image from the page and post it on SE? I'm not going to post an entire page, just excerpts.
Yes, it is fine to copy verbatim short passages of a book as a part of your question to ask for clarification.
This is preferable to posting an image. See this question for an explanation why: Should I edit a question everytime I see an image in it? (Of course, the above does not apply if you need to reproduce a graphic from the book. Yet, you should not post an image of text.)
However, make sure it is only a part of your question, and you add at least some actual question of yours to the copied text. Just "Please explain this to me." could be a bit to unspecific, and you might receive a poor reaction.
But "I understand the definition of group and subgroup. But in the argument reproduced above it says 'the subgroup $H$ is normal' and I do not know what this means or how it is relevant to the argument." is an alright question.