I used a slightly odd trick: I have an old Wordpress.com account and when you upload images to the asset library there, you can use the url you get. The image doesn’t actually have to be live on your blog anywhere, just in the library. You don’t have to worry about the file going anywhere then, since you still control it.
If you’ve got a Dropbox account already, then the above suggestions are good, but if you don’t, but do have Wordpress, this is a pretty easy workaround too.