A way to generate a universal image URL that works between Mac and iOS (DropBox, iCloud, other?)

I'm looking for a way to make all my markdown file universal that is work on iOS, Mac, even Linux/Window/Android. I want to have a single folder where I stick all my images and then point to that folder from whatever OS/device I'm using. For example

~/img/image.png

would be readable in the .md file from anywhere. Does anything like that exist? Can I use iCloud or Dropbox for that? Are there any other solutions people may suggest?