You would need to upload the image to your webserver (or some other place) and the url to the image is the exact location of the image, where it is stored on the server. For example if you go to Google.co.uk and right-click their logo and go to view image, you will be taking to a page that just displays their logo and the exact url for the image is:
http://www.google.co.uk/intl/en_uk/images/logo.gifSo the image is located on the Google server with a folder structure 3 deep (example below)
Domain Name
Folder Name - intl
Folder Name - en_uk
Folder Name - images
Image Name - logo.gif
Just upload your picture to your server, if you put it in the top level directory of your server, your url would be something like
www.yourdomain.co.uk/imagename.jpgHope this helps