When you sell a painting, does that mean you just sold your copyrights?
The answer is No
Unless you the artist, intentionally sell the rights, you still own them. This way you can make prints, license your designs and publish as needed.
What do you tell your customers when they buy a painting
Tell them in person, or when handing the painting over include a letter of authenticity, with a note that you retain all copyrights.
How does this apply to commissions
Before starting a project, have a discussion with your client, depending on the project and clients expectations, some will be flattered and it's not a problem for you to retain the copyrights while others may be commissioning you for a product or other specific reason where they want to control how the image is used.
Depending on the type of commission
Once you are hired, it is really is not so much your idea any longer, you are collaborating, so be clear with your client, and get it in writing before hand.
What if you take a workshop or online course
The composition, referance photo and all teaching materials are the intellectual property and belong to the original instructor.
Now here is a subject that isn't much fun to talk about, Copyright infringement, it never makes anyone feel good. Most people are not sure what was is and isn't acceptable, basically it simply means that you can not profit from another persons, intellectual property.
I know some people will get really upset when talking about this, but this is basic copyright law and these are the facts, you can check with any seasoned professional artist, organization or attorney. If you plan to "sell or show" your work it needs to be entirely your own concept, reference material and creation, It's okay to look in magazines, or websites etc but don't copy them (that goes for picture or paintings), when you have an idea, change the elements and turn them into your own.
If taking workshops or online courses, when specific lessons are given, the copyright is still the intellectual property of the instructor, not the student.
Lessons are meant only as learning tools to help you understand the thinking process behind the painting, and as you learn how to gather your own reference materials you will then be able to interpret them into your own paintings. For painting competitions, it must be entirely your own creation, in the show prospectus you will see that the organization will not accept work that is done in a workshop, online course, under any supervision, or someone else's reference material, it must be entirely your own.
As artists we all learn from imitation, and for those workshop paintings, you can give them to friends and family but don't profit from them, (basic copyright law). If you want to donate it ask the instructor and then credit them and the workshop.
Instructors spend many years perfecting their art and lesson plans, so if you decide to take your art to the next level, do it with your own creation just like they did.
If your intention is to sell paintings and you like someones else's photo ask them for permission to use it or license it for a fee, and always get it in writing. As far as competitions go, most organizations will say they in their show prosecutes that they want everything to be your own creation, from concept, reference photo, to painting.
For more information visit https://www.copyright.gov/help/faq/index.html