Simply connecting your phone to your computer using the same USB lead that you use to charge it should make your phone show up as an external drive on your computer. (Some phones require you to tap a button to set it to the appropriate mode but many/most don't). You can then simply drag-&-drop (or copy-&-paste) the image file to a suitable location on your computer.
Alternatively email the picture to yourself and download the attachment from the email on your computer.