I disrecommend this practice for text and equations. It will save you a small amount of time up front, but may cost you more overall. First, your ability to search is compromised depending on how much is a graphic. I search fairly frequently, especially when I want to update a card. That's the second disadvantage: you can't update a card so easily now. There's a third disadvantage: Typing up a paragraph from a book (in your own words) has additional learning benefits.

A fourth disadvantage is that different sources may have different notation. I've struggled to keep consistent notation for a number of my decks. You don't have much of a choice in the notation if you just take screenshots. It is a good idea to have multiple notations in your decks, but my suggestion would be to have consistent notation for the main cards and have extra cards about the less common notations.

In my experience, deciding what to add takes much more time than adding it. I make this as easy as possible by noting what I think is important when reading, reviewing, etc. I have lists of text files of things to add along with where to find more information. I have a Beeminder goal to add 3 cards daily that I don't mark as easy right off the bat, and I generally think about what would be most useful to myself and take them from my text files. (What I add tends to be technical, e.g., I am relearning combustion theory at the moment. 3 a day is reasonable for this, but the rate should depend on the subject and how much time you can invest.)

Graphical things are okay to take from books. However, I will frequently make my own graphic for the learning benefits and because I want to tweak/improve what I see.