There are techniques in some of the books in
namely Chapter 1 of
http://web.mit.edu/alexmv/6.037/sicp.pdf
but beware, that book is best eaten in small bites.
If you do the download right, like you did, without editting the image, you can drag the .png file into a Blocks Editor workspace and AI2 will use the hidden text in the file to recreate the blocks.
I save .png files of all my reusable procedures, by procedure name, to avoid having to recode them in other projects.