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.
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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.
