Would it be hard to Frankenstein a two 2 in 1 screenshot image file with an embedded rom?
So if you load it on a web site, or in a picture viewer, just the image would show, like a normal PNG.
But when you load it in an emulator, the embedded rom plays? No nefarious reason i ask, totally not.
You can do it with PDF files, since they will look for the PDF header after the NES header, as seen in
this thread.
PNG files don't allow you to put in anything before the header, neither do JPEG files. Although Irfanview will still view the images if there is garbage data before the image header, Google Chrome will not.
RAR files do allow you to put arbitrary data before the RAR header.
Is such the case for all browser supported image formats?
Zip can be appended to another file, as its central directory is at the end of the file. This means anything built on Zip, such as LibreOffice and Microsoft Office XML formats, can also be concatenated to an NES program. As for "browser supported", you can write a decoder for just about any format in JavaScript, though you can't embed it in a blog, forum, wiki, etc. that restricts what script an author can add.