Booting Linux distro from usb memory card

Hello,
For some reason booting operating system from usb isnt an option.
What am i doing wrong? Are my ISO images correct?
And how can i check if my iso images are correct?
Or do i have to alter something in my bios so i can acces my linux distro through usb?

Ps, i have windows 7 proffesional.
PPs, i have the usb card insterted into the receptacle before starting my computer