how did the seme get the picture of him in the first place?

It's photo shopped. If you think my answer is unreasonable, just compare the dick size ┑(￣ V ￣)┍

Yes just like Wolf says it's photo shopped. After all the uke did said with a smug face: "That's huge". (≧∀≦) Why should he point out himself that his 'own' d*ck is huge? Haha, unless he wanna show off, but he's not the type for that.