The Executive Summary to the Scope and Guidelines states
2. Reproduce the problem statement accurately. ...
4. Do not simply post images of the problem statement or your work.
The Executive Summary was posted when the site was very new. Since then the practice has been to allow questions and answers which are posted as images. This has not created any practical difficulties. It does mean that the question is less likely to feature in internet searches, but I think that is outweighed by the advantages of being able to see the exact text of the question (which fulfills point #2), and the time saved typing the question.
I think the only concern is that it might be illegal and make the website liable to a claim for compensation.
Copyright Law allows you to make limited reproductions from textbooks for personal use without the need to obtain the author's permission. This is known as the "Fair Use Rule" and probably applies to uploading images of text or illustrations from a textbook for the purpose of discussing the solution. Where the use is educational and not-for-profit, the amount copied is a small fraction of the book (usually 5% is the limit), and sharing is limited, you are probably ok.
See When can I use a work without the author's permission?