What image & compression format is needed to sideload a tile package into an Android tablet for offline Collector use? It didn't like the ecw format I tried.
The only format for this purpose is the TPK (Tile Package format) as described here:
Using your own Basemap layers with Collector for ArcGIS | ArcGIS Blog
You can create these files in ArcGIS for Desktop in several methods as described in my blog post:
Create Cache and TPK without ArcGIS for Server - Workflows for Desktop users
The usage of these files in the collector is as Basemap.
Retrieving data ...