I have a great old photo of my German Shepherd. I scanned it in high res 1,200DPI.

Thank you very much for taking your valuable time to offer me a reply. Very Much Appreciated!

Look at what your competition is doing in the same category. 

https://stock.adobe.com/search?k=%22german%20shepherd%22

Major looks like a great dog!  However, Stock customers expect highest visual and technical quality for those million dollar print and digital commercials.  Unfortunately, the image is not suitable for Stock.  

• https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
• https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html

Better luck with your next submission.

Yes, you could clean-up the dust and scratches and crop out the borders. But pictures like this also have that nice special grain that some try to reproduce digitally. But that grain makes it unsuitable for stock. In addition, you would need to correct the colour cast that is adding to the charming look, but again is not suitable for stock. A last problem is that the dog is not crisp sharp as we are now used to see in the digital age.  There is not much you can do to safe the picture for stock, but that does not diminish it's charm.

I have a bunch of my slide pictures scanned and published on flickr and they are doing very well. But I could not even think to publish them here, they would not oass the quality check.

Stock is not about art, it's about craft. Your pictures need to be clean and sharp, with very low noise, so that the buyer can use them for any purpose he wants, even for applying fancy filters to give the picture that nice vintage look of yours. 

Sorry about that, but I'm happy that I saw your nice picture. It deserves all the views it got.