What Causes Visible Vein On Penile Head After Having Erection?

Hi, I have noticed that after erection there is like a vein on the head of my penis and it looks like this

(this one is not mine)

http://www.imageupload.co.uk/images/2015/01/10/vd2.jpg

this one is mine

http://www.imageupload.co.uk/images/2015/01/10/DSC0318.jpg

http://www.imageupload.co.uk/images/2015/01/10/DSsC0022.jpg

http://www.imageupload.co.uk/images/2015/01/10/DddSC0016.jpg