There's a similar but not-quite-equivalent discussion here: Exporting symbology from ArcGIS Desktop to ArcGIS Online?
The people in that discussion, from 2011 to 2014, were using AGOL and trying to upload layers with symbology included. I'm using a free individual account and trying to upload a custom image to symbolize a layer already displayed. The interface seems to offer that option, but it doesn't respond as it should:
I've clicked on the "use an image" link indicated in the screencap. In most interfaces this opens Finder, Windows Explorer, or similar. In this interface all it does is open an input box. I've tried pasting in the entire file path to Marker.png. However, clicking on the blue circle containing the white plus sign, I consistently get:
"Unable to load image."
The image is a simple .png with transparent background, 120 x 120 pixels. I've tried Chrome and Firefox. Anyone have any ideas?