This page will explain how to upload an orthomosaic file to your account on ArcGIS Online (AGOL)
Right click the image layer and select Share -> as Web Layer
In the "Layer type" section, select the “Tile” option
In the "Share with" section, make sure that you share the new layer with the appropriate group.
Go to the “Configuration” tab and make sure that the “levels of detail” slider is set to its maximum value (level 23) and that the "Allow clients to export cache tiles" checkbox is turned on