I haven't tried this yet, but this is exactly what I was looking for! Xam, I don't know if you already figured it out/stopped trying, but here's what I do for a picture already taken (Note: this only works if the picture is taken with the app you are making.) When the picture is taken, I also save the URI in WebStorage. You set a local key (ex: "Picture" & PictureCounter ...or whatever string you want to create) with the value of UserMedia.SnapshotURL.
(This part is an educated guess, as I haven't done this next step)
When you later want to upload the picture, in the AJAX action, replace UserMedia.SnapshotURL with WebStorage.LocalValue("Picture" & PictureCounter) or with whatever your string is.
If you want to upload a picture taken with the phone's own camera app, I do not know how to do that yet, sorry.