how to render jpg?and I use this jpg to print
You may have to use photoshop to convert your unreal screenshot! Jpeg is lower quality than the output format unreal gives you.
Depending on how you take the screenshot, it’s not possible to convert it to jpg in the UE4 
e.g you could render your screenshot with matinee (the record movie setting) + choose jpg. Otherwise just do it with the high res command and convert your pic to jpg in photoshop or gimp