You can host it here on salvation. Whenever you go to create a new post or edit an existing one there is an "Upload an Image" button at the bottom left of the box you type in. Using this you can upload your signature picture onto salvation in a post. As an example I attached an image to this post. From this point you can right click the image and "open link in a new tab." From here you have the url to the image you attached.
http://media-dominaria.cursecdn.com/attachments/164/231/636348677427874572.jpg =
If you dont want to do this, you may use any image hosting site such as Facebook, ImageShack.us or Photobucket.com as examples of our accepted sites.
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[Modern] Allies