WorldMap Warper as well as NYPL Map Warper are great tools to georeference a map online. Tonight I spent a bit of time try to load a rectified map to AGOL. This is the map in AGOL.
Here the trick:
1) Get the WM Warper URL (http://warp.worldmap.harvard.edu/maps/tile/31/z/x/y.png)
2) Change the last z,x,y to {level}{col}{row} [http://warp.worldmap.harvard.edu/maps/tile/31/{level}/{col}/{row}.png]
3) Load it as Tile Service (http://blogs.esri.com/esri/arcgis/2013/03/21/adding-tile-layers-to-your-web-map/)
