Michael,
In ArcGIS Pro you are Sharing as Webmap?
If so, click Configuration
A tile service will be created for the Image layer in your map... Click to highlight it, and click properties.
Then click Configuration for it.
This will show you at which scales the layer will be view-able, check to make sure this works for you.
Is your Collector user going to be working offline?
There is another option for putting a basemap on the collector device that can be used with the editable webmap.
Go offline—Collector for ArcGIS | ArcGIS
Basically, in ArcGIS Pro you can use the Create Map Tile Package tool, to convert your JPG into a tile package (.TPK) The tile package can be put on the users device, and be used as a basemap.
Mark