If you ever accidentally run across actual child porn and not just kids in bathing suits remember you need to report it to the police immediately to avoid getting into trouble yourself.
Whatever you do don't try to access the file or page again once you report it. Immediately when you see it call the police. If you access the file or page again voluntarily then that can be held against you. If you see it on a web page then the image is probably cached in your computer's browser memory and this can be very bad.
You should probably clear your cookies as best you can, delete your browser history, etc. without directly accessing the file. Report what you saw to the police and where you saw it. Do not return to that web site, even to check on whether it was taken down. Going there again could cause you to get into trouble.
Even allegations that you had that happen and didn't report it or voluntary accessed it could ruin your life and career so if you ever see anything like that you need to be very careful, report it immediately to the police, and clear your web history/cookies.