Photo vs picture vs image: what is the difference?
Short answer: every photo is a picture, every picture shown on a screen is an image, but not every picture is a photo.
Photo
A photo (photograph) is made by a camera recording light — on film or on a sensor. A selfie, a scan of a printed photograph and a passport photo are all photos. A drawing is not, however realistic it looks.
Picture
A picture is any visual representation of something: a photo, a painting, a sketch, a cartoon, a diagram. It is the everyday word, and the widest of the three in ordinary speech — “draw a picture”, “the pictures in this book”.
Image
Image is the more technical and formal word. In computing every picture on a screen is an image file — JPEG, PNG, WebP — whether it began as a photo, a drawing or a screenshot. Image also has meanings the other two lack: a reflection in a mirror, a mental image, a brand's image.
Which word to use
Say photo when a camera made it, picture in everyday talk about anything visual, and image when you mean the file or are writing formally. On this site the three are used for the same thing: whatever you drop into the two boxes.
If you came here with two pictures to compare rather than two words, the comparison workspace shows what changed between them, and the compare-and-contrast worksheet turns two pictures into a classroom exercise.