I like how e.g. Cassandra Calin does it. Similar to what @joannekwan does, the signature is part of the image but always very decent and subtle. See for example this or this.
Not intrusive and very effective for those interested in the art and the author. Just make sure you turn up in the results when someone types your name or alias into a search engine. You can't prevent people from stealing it but I guess the key is to prevent people from being able to easily screenshot your image without your signature. @zigmenthotep's example would be easy to cut off at the bottom to exclude the name.
My recommendation would be to add your Instagram name as a signature to the images.