Put the location of CentOS image into Image File text area. Win32 Disk Imager can accept CentOS 7 *.iso file. No need to convert *.iso file to *.img file.
Choose your USB drive that you will bootable CentOS 7 as the Device.
Click button Write to start creating bootable USB drive.
When finished, boot the desired pc that you want to install CentOS 7. Make sure the created bootable USB drive is already attached to the desired pc.
Setup bios to boot from the USB drive.
Make sure USB Drive Emulation Mode setting in BIOS is set to Hard Disk. Not Auto or Floppy Disk.