Very generally, the creator of the artwork automatically has license to control its use and reproduction by everybody else. Ignoring those rights can have legal consequences (plus it’s unfair to the artist).
Most websites that sell images commercially will have a specific section regarding how the image may be used (often under the hyperlink labeled “license”). Quite often they are free for private use but restricted from commercial use. Most commercial websites will also have the costs of those various licenses available as well as a description of use under that license. For all such images we will require, at a minimum, proof of a single-use commercial license. The artist has the absolute right to set the price at what ever he/she wants. We have found that most artists are very reasonable for individual/limited reproductions.
Fortunately there are also a number of artists and websites dedicated to providing “totally free images.” Whether referred to as public domain, creative commons, or CC, these images have been granted public copyright license by the artist and are free to use privately and commercially. www.pixabay.com is one such website that we use quite frequently.