Question this question is about;
Where can I post my pencil drawings online?
Is this on topic for A&C SE?
If not, do you know of another SE site where it would be on topic?
Arts & Crafts and other SE sites
A&C focuses on the "making" aspect, so questions about creating the drawings would be on-topic. Advice about online sites for posting the drawings wouldn't be on-topic.
As far as alternative sites, as a general rule, all of the SE sites have an objective of building a knowledge base that has long-term value. Information like what are good web sites or products is short-lived. What's a good choice today may not exist tomorrow, or tomorrow there could be much better ones, so the information quickly becomes outdated.
Also, implied in the question is what are "good" sites (you want recommendations, not a regurgitation of every random site found in a Google search). That gets into opinion. Everyone has a different, equally valid opinion of what sites they like. Simple lists don't really add value beyond doing a Google search (which people are expected to do before asking a more targeted question; questions shouldn't request that other users do their search for them). So that isn't a good fit for the SE model.
SE sites distinguish between questions that request general-purpose lists without qualification (generally off-topic), vs. questions that define detailed requirements for a specific need and request recommendations that include "why" each recommendation is an especially good solution. Those kinds of answers might not always have long-term value, but at least they fully address someone's actual current problem.
SE sites vary a little on where the line is drawn on the types of questions described above. As you phrased the question, I think it would be off-topic on virtually all SE sites. Some sites allow very targeted requests for resource recommendations. If you were asking about resources for creating the drawings, a carefully crafted question is sometimes on-topic on A&C. But posting the results online basically falls outside the site's intended scope. So even if it was a very targeted request, it wouldn't be on-topic for the site.
In this case, SE may not be the best resource for what you're looking for. I would suggest starting with a Google search and visit some of the site recommendations you find. That will provide a starting point. From there, it might be useful to engage in discussion about the relative pros and cons of the sites you've found, and alternatives you might have missed.
SE doesn't really provide a good venue for that kind of discussion, other than the chat rooms in some cases. Open-ended discussion is better accomplished in a forum format rather than a Q&A site. So in addition to doing an initial search for sites for posting your drawings, search for art forums where this can be discussed. You might even find a few where the topic has already been discussed, potentially providing existing answers.
Another approach is to search for reviews of sites. Try search terms like "best sites to post drawings". Those kinds of hits will get you close to what you're looking for.