Clean the USB disk:

Run cmd.exe as admin
diskpart
list disk
(see which disk number is your usb stick)
select disk #
clean

If you get "access denied", you may need to unmount the USB disk first:
cmd.exe as admin
mountvol (driveletter) /d
(e.g. "mountvol h: /d" to unmount h:)
...or use disk management "change drive letter or path" to do this.

Writing the image to the usb stick:

Run ImageWriter.exe as admin, select the image file, select the USB
drive, and click "Copy"

If the image doesn't show up in the "select image file" dialogue, try renaming
it to have a .raw extension.