Image conversion of user uploaded images - overkill?

I would like to know -

1. If it is a security overkill to re-convert all user uploaded images on a public facing website into their .png form (if possible) on the server using GraphicsMagick?
2. Will this conversion / security method scale well - I mean in terms of server load (running nodejs) on converting all images (website will be image heavy)?
3. Do pinterest, twitter etc. scan all uploaded images or re-convert them?

Thanks!

via vjjj