I think it would be useful to share an actual URL (rather than the data that a URL points to), and in addition it would be useful to be able to share the data for a file:///... URL (which is possible) and also for a data:... URL (which is apparently not possible currently).
According to the Share documentation, you can only specify a URL on iOS. You might want to upvote the following feature request, although I’m not sure it will completely solve your problem and even if it is implemented in SDK 39 you’d have to wait until September for that:
You can share an image using the Sharing module, but this only allows you to share a file. You can’t share a message in addition to the file. See the following snack for an example:
I had the same question, thanks for asking it and thanks for answering. Seems like I’m going to have to download the image first with expo install expo-file-system