Go to the My Content area of ArcGIS Online. Click the Add Item button. Browse to your tif file and add it. This is not adding it to a map but just as an item. Then you will have to add the tif file to your map. When you are in the map, click on Add (toward the top left), then Search for Layers, then click the Go button. The tif file should show up in the list. Add it then wait a little while.
If your georeferencing and coordinate system are correct, I assume it will fall into the correct place.
Paul Lohr
