Hi Emily,
There are a few ways you can go about emulating the Restoring Old Havana cartography. The maps in that story are actually a basemap called Seahaven with about 50% transparency overlaid on top of a parchment texture image. We built them in ArcGIS Pro, and did the final styling in Adobe Illustrator. Unfortunately, you can't georeference directly in ArcGIS Online. But there are a few different ways you can create a similar antique effect in a Web Map. To get you started, here are two examples of how you might go about it:
First, if you're starting with a historic map that you have already, you'd have to do what you described and georeference the image in ArcMap or ArcGIS Pro on your desktop, upload it to ArcGIS Online as a Tile or Feature, add it to a Web Map, and overlay your other layers on top of it. Here's what the Havana maps would like like using this method https://arcg.is/0aPezy . This route gives you a little more control over the map image, since you can choose and style it as you like.
Second, if you want to start with an existing ArcGIS Online standard basemap and just give it a historic look, you can spoof the effect. On your desktop, start by finding a high resolution image of a texture of your choice, georeference it to the desired extent, and upload the file to ArcGIS Online. Create a new Web Map, and choose the basemap that you want to use. Then add your texture file to the Web Map as you would any other layer, but add about 50% transparency. If we had done the Havana maps this way, they might have looked something like this https://arcg.is/1WaOaP . With this route you lose some control over the look, but you retain the interactivity and dynamic scaling of the ArcGIS Online basemaps.
I hope I've understood your question, but let us know if you had something else in mind.
