Some free Skymap resources:
https://render.otoy.com/vr_gallery.php
The OTOY skymaps are stereoscopic. At least, I think that’s the term. My favorite is “The Dragon’s Nest”. I couldn’t find a license description but it’s a simple download. If nothing else, download the ones that you like for your personal use!
The ones on the github are ones that I created using Google Streetmaps. Please feel free to use/distribute any or all of them, as far as I’m concerned.
WOW thats freakin awesome, thanks a lot man - this will be golden for PE!! 
The format on the Github ones were altered for VRToolbox but I’ve got the originals, in case you run into any problems. Github is dds, I’ve also got jpg’s. Also, I believe that I had to reduce the quality to upload them. The originals seem to run from 5Mb-25Mb. As I say, let me know if you want to pursue them.
Edit: in particular, you need to test the mirroring. For VRToolbox, I had to invert them. I believe that you would need the same but I can probably invert the originals without sacrificing the quality, if needed.
(Welcome to Dr.Strange’s Mirror Dimension 
)
Thanks! Size is really not a huge problem, as long as its somewhere between 10-30mb each. Actually, many of the sky/cloud skyboxes that are included in PE were 8K resolution and each was 40-60Mb in size. I resized the textures to 4K and 10-25Mb each.