As an expert in Desktop Application, JavaScript, Python, and Web Application design, I bring a wealth of knowledge to your slideshow project. Over the years, I've developed a deep understanding of user needs and preferences, which I believe is crucial for a project like this. I'm greatly familiar with both Windows and Mac operating systems, ensuring an app that's seamlessly functional across different platforms.
With my robust skills in Python scripting, I will create an intelligent application that not only displays your 28 square images perfectly on the desired 16/9 fullscreen but also recognizes new additions to your Dropbox folder in real-time. The image presentation time selectability of 5-15 seconds and the fading transition between images are definitely doable within the project time frame without compromising quality or efficiency.
Moreover, with my experience in SEO, SMO, PPC, SMM, Mobile APP Promotion - all components of digital marketing - your app for event images will receive diverse promotional strategies to maximize its impact. Not only will your event's photos bring back fond memories for attendees but also give a boost to your brand through strategic promotion. In conclusion, my extensive skill set and sincere dedication make me the ideal candidate for this project. Choose me and let's make this momentous event truly unforgettable!