As an artist, it is generally not legal to use any image from the internet as a reference picture without permission. While artists can certainly use photos for inspiration and to learn about the characteristics of their subject, directly copying or reproducing an image would likely infringe on the copyright of the original photographer or creator.
However, there are ways for artists to legally use online images as references. One option is to use public domain images, where the copyright has expired or been waived. Websites like DeviantArt also have unrestricted stock clubs where artists can find images they can use. Additionally, as long as the resulting artwork is a unique interpretation and not a direct copy, using a photo as a reference is generally considered fair use. The key is ensuring the final work is transformative and adds something new, rather than simply duplicating the original image.