A rom image is like a picture of a video game that you can play on a computer or a game console. Think of it like taking a picture of a book and then being able to read the book on your computer without actually having to hold the book.
When the video game is made, the people who created it make a copy of the game's data called a rom. This rom is basically all the information that makes the game work, like the graphics, sounds, and code that runs the game.
Sometimes people want to play these old video games on their computers or consoles that aren't designed to play them anymore. So they use a rom image, which is a digital copy of the rom.
You can think of it like taking a picture of a book and then being able to read the book on your computer without actually having to hold the book. So, a rom image is just a digital copy of the rom that people can use to play the game on something other than the original console it was designed for.