It's hard to say if activating Google+ authorship on image sales pages is really a good idea at this point, since the feature is still fairly new and there's not a lot of hard data on the SEO results.  A couple of points to consider:

1.) This might modify the display of your image on Google to show your G+ profile image next to the search result.

This makes a lot of sense for a news story or blog article and Google says that it can increase the click-through rate (CTR) of the result - especially if you have a pretty face!  However, it's very possible that this can decrease CTR for ecommerce items, since at a glance the searcher might think the result is just an article or opinion piece and not something to buy.  My speculation is that these results could be unconsciously skipped when a buyer is looking for an item on a search engine, similarly to how our eyes have been trained to scan past ads without seeing them.

2.) This provides a claim of ownership on the image.

Although I don't know if they've outright stated it yet, it's very possible that Google will start using authorship in the future to collapse similar results and give higher priority to "claimed" pages and images.  In the case of multiple claims, it would make sense to give highest priority to the first authorship claim, although who knows if this is how it will really work.  With that in mind,  if you have your own portfolio site or self-hosted storefront, it's worth considering whether you would really want to enable authorship at an agency such as DT unless you can prioritize your contributing sites on your G+ profile.