I'm an (aspiring) professional writer but an amateur illustrator. I draw characters and scenes from my book strictly for my own amusement and to help me develop my ideas. They are not intended to be part of the published book. Obviously, I don't mention my art in queries or anything like that, but I'm wondering what is and isn't wise to do with this art. For instance, can I post it on my blog and social media, where it isn't directed at agents, but they're likely to see it if they research me? Or should I keep it under wraps to avoid looking amateurish? (For the sake of argument, let's assume that I'm a fairly good artist.)
You haven't mentioned what kind of books you're writing. If you plan to write novels for adults, art work won't help much at all, since most of those book don't contain any illustrations.
If you're planning on writing for kids, illustrations are used in varying amounts depending on the kind of book: picture, early reader, middle grade etc.
Art is a very tricky topic. Some really awkward looking things can end up as phenomenal successes:
And some images are so wonderful they just grab your heart the second you see them and never let go:
And some aren't cute or funny, but they really make you want to read the book:
But in the end, this is your art, and your blog, and if you want to share your work, you should. You can't predict what anyone's response will be. I can't imagine an agent looking at your blog and saying no to a project that was otherwise a yes simply because s/he didn't like your art. (although, I do have very strong feelings about certain fonts....but that's another blog post.)