I'm using the Export Tile Cache tool in ArcToolbox to create a TPK file which is then sideloaded into Collector iOS using the methodology described here: http://doc.arcgis.com/en/collector/ios/collect-data/offline-use.htm
All this is working fine but I noticed that when using the Manage functionality within Collector to look at the TPKs that it shows a default thumbnail for each item.
It would be quite useful if each TPK could have a custom thumbnail image to help users distinguish between the different offline basemaps. If I poke around in the TPK file on a PC then I can see that there is potentially provision within the esriinfo\iteminfo.xml file for a thumbnail e.g. <thumbnail>thumbnail/thumbnail.png</thumbnail> but there doesn't seem to be any kind of workflow that I can find to actually set this to a custom image.
It's really not a major problem but thought I'd post just in case anyone else had cracked this particular nut.
So glad to see the the ESRi community is so eager to reply to your simple question about generating thumbnails. Since no one here seems to be able to answer your question, might I ask you if you EVER figured out how to generate thumbnails? Thanks