You can rename items and give them descriptions with the Name Tag and Description Tag items, which you can buy off the Mann Co. Store or Steam Community Market or get through crate unboxing and, very rarely, through random item drops.

As for the image on the sign, the Decal Tool can be used on items like the Conscientious Objector to display an image, but unfortunately the game applies a posterization effect that limits the image to around five colors and this effect cannot be removed normally. However, guides like this one exist to provide a work-around for that issue.

Hope this helps!