As a user who would also want this feature, I would say it is because you sometimes want to see full detail. If, for example, someone sends you a link to a painting or some old coins you might be buying or whatever.
There's now a link next to the "Download Photo" button called "View Original" when you click to view the larger photos.
Update
Unfortunately on Chrome this link doesn't work because the photo is stored in a way that tells Chrome to automatically download the photo. I'll have to dig in to figure out if there is a way to force the photo to be displayed instead of automatically downloaded.
Are your originals actually the original files? Some people have strong opinions about data leakage via EXIF metadata, specifically location and sometimes date/time. Do you strip that information out during upload/resizing?
Overall, it looks great. My family would definitely use it, for similar reasons to yours. Nice work!
The code/library/service responsible for storing the original file on S3 is most likely setting "response-content-disposition" to "attachment" when it stores the file on S3. That tells S3 to deliver the file with "Content-disposition: attachment" in the HTTP headers which is what causes browsers to save to disk instead of display.
You want "Content-disposition: inline" or no content-disposition header.
It's mostly due to the fact that the scaled version is only the middle ~50% of my 1920x1080 screen. I suppose what I might really want is scaled, full screen, and original.
Out of curiosity why do you want to view the full-resolution photos vs. the ones that are scaled to fit your screen?