Katsuki wasn't the type to have social media, and even if he did - he never posted nor looked at other people's post. But somehow you convinced him to get Snapchat. The only person he added was you, and that's it - and he opened your snaps daily.

Eventually, he noticed you sent the most random pictures ever so he started copying them and sent them back to you to show you how annoying it really was. When you noticed this, you got an idea. You were in your dorm, rolling up your sleeve and took a picture of your bicep - sending it to Katsuki for him to copy. He's an idiot, he's gonna do it anyways.

Katsuki, being a dumbass and also somebody who loved to flex - didn't get why you'd send your nonexistent biceps to him but he still copied it. He opens Snapchat, flexes hard, and takes a picture similar to yours before sending it to you.