I did a google image search for creative commons "botanical illustrations" and came across "Paxton's Magazine of Botany and Register of Flowering Plants (1838)" via commons.wikimedia.org.
I did a super quick selection using magic wand, which as you can see below would not be great for print-ready artwork. But it works in a pinch!
I left the composition of the original drawing as-is and added a background that was very close in value and color to the original image. That way, any remaining bits left over from my magic-wand selections would blend into the background color.
By adding a subtle drop shadow it makes the white text pop!
This is such a fun class! I would recommend it to intermediate beginners and seasoned design professionals.