The D&D Beyond versions of the books include all art from those books
The compendium content of each adventure on D&D Beyond is an online browsable version of that adventure. It contains not only the adventure text, but also cross-links and tooltips for monsters, mundane or magical items, spells, and relevant rules mentioned in the text... But most importantly for this question, it includes all the art and maps from that book as well.
D&D Beyond co-founder Adam Bradford confirms that the books on D&D Beyond include all the artwork from that book in this forum thread. As he notes in the thread, the artwork may not appear exactly where it does in the physical book due to different layouts and the like, but they should all be there.
And on the note of maps, the DDB versions almost always come with both DM and player versions of maps, even when the physical adventure book only contains the DM map within the book itself. I believe these maps are provided to them by WotC themselves, so it is a plus compared to the physical books - all maps and art are available in full resolution.
(I don't own the books on other digital platforms, such as Roll20 and Fantasy Grounds, so I can't speak to whether they also come with the art or which art they include.)
However, this art is copyrighted by Wizards of the Coast. I don't know whether any of it is allowed for non-private use or distribution; I suspect most such cases are not covered by fair use or similar. That said, I am not a legal expert, and that is a separate question.
Related: How to get permission to use Monster Manual images in articles and adventures “courtesy of Wizards of the Coast”?