it's my understanding that new users can't post more than one link, in order to avoid SPAM. I assume the same is true for images. Can/should we as editors override these restrictions for in cases where we feel it's beneficial? For example, adding a second link here or linking the two screen captures here?
I'd say, definitely.
Yes, it's fine. As you say, the reason for prohibiting low-rep users from posting links and images is fear of spam. If you read the question and decide that it's okay to post the link/image, then there's no fear of the link/image being part of a scripted spam attack*.
*Unless you are Skynet, in which case we have bigger problems anyways.
I believe one drawback to this is that, if the new user then edits their own question, the images or links may be removed again if they still don't have enough Rep to avoid the restrictions.