When posting images online, it's essential to always give credit to the original source of the image, whenever possible. Not only is this respectful of the original photographer, illustrator or other creator, but it also ensures that their work is attributed and that they receive recognition for it.
The most appropriate place to put photo credits is beneath the image, with the format being "Photo by [Photographer's/Creator's Name]". To go the extra mile, you can add a link to the photographer's website, or the source of the image.
No matter where the image is posted, it should always include the original source. If a photographer or creator is not known, "Image Credit Unknown" should be included.
In short, the photo credit should be given and should be clearly visible and easy to find. Giving credit where credit is due will ensure that photographers and creators receive the recognition for their work that they deserve.