Hey Wendy,
You can publish your own symbol to ArcGIS Online and use them in a symbol gallery so they are accessible to use anytime and can be used across multiple accounts. This blog covers how to publish this symbol group (web style) to ArcGIS Online, it focuses on using vector symbols but your JPG can be used as well. A vector symbol will just give you a sharper symbol as it gets sized and looks great on high resolution displays.
https://www.esri.com/about/newsroom/arcwatch/create-2d-symbols-and-publish-them-to-arcgis-online/
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