I had the same issue as well. I believe it is new IT policy at work that was preventing uploads from Windows desktop client to web-based EN and thus my iphone as well. Work around is to do the the screen captures and uploads of PNGs from web-based version allowing the PNG to upload and synch to desktop version and iphone:
Description of problem:
For about a week now, PNG files that I drag and drop into EN on my windows desktop do not synch to EN on the cloud, nor to my iphone. I shared the notes from the desktop to my partner's phone and the pngs do not open there either. So, in all of the above cases the note itself appears on each client, and all of the text syncs, but any PNG that has been inserted into the note appears only in the note you originally pasted into - On all the other devices you sync to, it is just spinning blue circle where the PNG should be.