You would use 'in' for both the text and the pictures. You would use 'on', for example, for on the front or the cover of a book or magazine.

Use "in" when it's part of it (like words in the text or people in the picture) and "on" when it's something in front, (like an insect landing on the text/picture) If you said a secret message appeared in the text, then it would use the words somehow. If it appeared on the text, it must come from somewhere else (maybe protected onto the page you're reading).