The built-in Photos, Mail, Notes, Messages, Books, and Files app let you annotate and doodle on pictures using Markup tools.
You can use a pen, highlighter, and pencil tools to draw or write on them. You can also add shapes, arrows, text, lines, signature, speech bubbles, magnify a part, and more.
This comprehensive tutorial shows you how to use the built-in drawing tools to Markup photos on your iPhone and iPad.