There are few things to check if you don't see your photos in Carousel or in your Dropbox account.
If the photo and video files are fully synced with Dropbox, then only you can see them in Carousel. If your photos are fully synced with Dropbox, but you still are not able to see them in Carousel, then check the "Missing Dates" section of your Carousel. Many photos that were not taken with a digital camera will be missing date information. Those photos will appear in the "Missing Date" section of your Dropbox and the Carousel. If you want to access the "Missing Date" section, then simply scroll all the way past any photos that do appear. The "Missing Dates" section is always next to your oldest pictures. Dropbox offers 2 GB of free storage space to all its users. So check the storage space for your account to hold your photos. If you have more than 2 GB of photos, then you may need to get more space to sync all your photos.