Post every day! Post multiple times a day!
Don't know what to post? As a creative you have so much content. What you do is inherently appealing and visual, so go ahead and post it.
If it doesn't get that many likes, it doesn't matter, don't be tempted to archive it, just keep moving forward and posting more. Learn from your poorly performing posts, do a self-audit. Was the lighting bad? Boring subject matter? Too sponsored looking? Too sales-y?
Being a creative is vulnerable, it's courageous to put your highly personal creative work online for the whole world to see it, and everyone thinks their own art sucks (that is normal). You have to find a way to break through the fear of judgment (again, normal, everyone feels it) and not dwell on what people might say, or whether they'll like it.
The Instagram algorithm is consumer oriented. It is showing people what they want to see based on their behavior in the app, not what YOU want your followers to see.
Now that Instagram is mature and monetizing, organic reach is declining, which means that your followers are not seeing all your posts, as sponsored content is now being prioritized. This means that you have to be even more intentional with your strategy, to break through the noise, as your content is being seen less and less. So, post more. The most recent statistics suggest that only around three to six percent of your Instagram followers are even seeing your content. Unless they hit like on every post, which most people don't do, your content gets lost in the vortex, so you may as well post more. Your true followers who genuinely want to see your content, well, they want to see your content, so they would probably be stoked to see three posts a day from you.
What time should you post? Morning and afternoons are usually best, but don't worry too much about this, views can vary greatly regardless. I go ahead and post usually in the mornings, but often just when I feel like it, I don't fall in to the habit of stockpiling content to post 'when people will see it', as I then end up with too many pics and videos on my phone, overwhelmed, and then I don't post any of it!
Here are some suggestions for content: Close up pics, pics of edges or interesting angles, portrait mode, your signature, your studio or work space, your tools, books, sold work, selfies of you working, pics of you in the field, urban sketching, plein air painting, at shows and exhibitions, pics of things that inspire you, your art staged in a room, customer appreciation pics, your pet in your studio, inspirational quotes, throwbacks, a screenshot of any website that represents your work, your work on merchandise, the finished piece on an easel or a wall, other peoples art, interviews and articles, and videos.
Which leads to lesson #2