Sure! It will be written just like any other json string NovelAI uses. Also, you can pick any PNG image as a storage, "cards" are only convenient for quick labeling.
Haven't received any reports of incompatible browsers, but that might be the issue. Which one do you use? If it's Safari, check the earlier comments for possible solution.
Itch.io may experience rare downtimes, clear cache (Ctrl+R) to make sure the page is loaded properly. PSD was tested on SAI and GIMP, are you sure there is a single layer and not a group that can be unfolded?
From what I could find out, this error may be caused by disabled third-party cookies which is the default setting for Safari. I don't know how to set up exceptions, but you can allow any cookies under 'Privacy' tab in 'Preferences'. Don't forget to refresh the page afterwards.
