I suspect there is more than one place such images are drawn from. Most anime series come from some source -- manga, visual novel, video game, etc. So if the team building the OP sees something they think will fit well, it is already there, and just needs to be redrawn in anime form.
The YouTube channel Mother's Basement has an ongoing series called What's in an OP (and What's in an ED) where he analyzes the OP or ED of a given series. In his OP analysis of Mob Psycho 100, he mentions several scenes that were from the manga and never even appeared in the anime. The images were just colored a bit with minimal artistic changes.
Other series are anime originals; the anime is the source material. The team may have the plot written out and story-boarded. A scene from a story board might be redrawn and added to the OP just as if it had come from a manga.
At least recently, some series have their episodes broadcast almost as soon as they are done, or even before. For example, I read on various boards that episodes of Yuri on Ice were being aired so quickly that what Crunchyroll got was not even the final episode version. So for that series at least, the animators would not have been able to access scenes from future episodes to throw into the OP. (Of course, its OP and ED were such that they did not need those kinds of scenes.)