How to detect if post includes a graphic?

I’m not sure if this is the right sub-section for this, but since I want to add this into a theme, I figured I’d roll the dice.

I’m looking for a way to detect if a post has a graphic included in it or not and load a different style sheet, or apply different styles to the entire page, based on that. Honestly, I’m not quite sure where to begin! If there’s a theme like this already that someone can point me to, I’d appreciate it. Or, any suggestions for where to start with code would be appreciated, also.

Either way, thanks in advance!