If I purchase a professional photograph - am I then allowed to treat or adjust that photo any way I wish once I own it? For example, would it be an infringement of the copyright to enlarge the photograph and apply a special glossing effect for my personal use if I feel it will add to the viewing experience? This is not much different from choosing the correct mat and frame for the image I would guess. But need some verification.
If you purchase a professional photograph (as in a print) then what you get to do is display the goods you've bought so that you and others can look at it, admire it and enjoy the memories/feelings associated with it or thoughts it provokes. At some point you may even ponder everything the photographer put into it to make it an image you love enough to copy. You of course can also sell or give away this marvellous work of art to another person who may get as much, if not more, enjoyment from it than you have. What you may not do is reproduce it as an entity in it's own right, it is the author's right to copy the image and decide who else can copy it.
Some photographers take exception to people producing poor quality copies of their work (by scanning it on a crappy home scanner, possibly adding some crappy effects and printing it out on their crappy inkjet at home) which may reflect badly on their reputation. Though for the most part, for personal use it would probably not be worth the photographers time to chase you for it if they knew about the infringement.
The legal side varies by region but most likely they'd be legally within their rights to seek some kind of redress if they so desired if you reproduced their work without permission.