Yes, if something is illustrated it means there are pictures. But there can be illustrated editions of the full text or than can be children’s/abridged illustrated editions.

I had a children’s illustrated Moby Dick when I was in 3rd or 4th grade and didn’t realize it wasn’t the “real” Moby Dick until I went back to it in high school. It was this one. It seems like there is a Frankenstein in the same collection, I’ll bet that’s what you read as a kid

If you read and understood it in third grade, I’m guessing you read a Children’s edition. Which probably took a more straightforward narrative approach rather than using the letters that Mary Shelley used as her narrative device