This would be difficult to conceal, for a number of reasons:
Most image formats used these days have well-documented formats, and are interchanged between different companies' software, so extra fields would be difficult to conceal
Lossless formats (e.g. PNG, GIF) would by definition not allow such modification; in practice it could be done, but it's likely users would notice
Lossy formats would allow such seganography, but there are likely too many formats and too many software companies for there to be any concerted effort that has yet to be publicized.
On a separate note, many (most? all?) manufacturers of color printers DO print almost-invisible pattern of yellow dots on each output page, encoding information about the printer, probably as an anti-counterfeiting tracing device