Hi Jack!
This could be a number of things. I'll list the most common:
[1] An HTML tag isn't closed correctly. Check the tags after the first image and before the second one...including the IMG tag itself. This is what typically causes the issue.
[2] There is CSS code or some other command overriding the centering. Check to see if there is an alignment inside any CSS commands that the IMG is inside.
[3] The two pictures are interfering with each other. Try putting the two pictures in separate <center>, <p>, <div>, or <span> tags so that both have centering commands.
[4] Simply typo. You command to center the second picture is misspelled.
Good Luck!