Note the several photo albums, many of which hold photos from past reunions.  You can view an album’s photos in either slide show or album formats by clicking the appropriate button. 
You will see two albums for the 50th: Nostalgia and Recent Photos.  Go to the one that matches the photos you want to share.  You will upload them one at a time.  For each photo, click the Upload Your Photo button, navigate to your photo on the computer, fill in the requested info, and click Upload.  (Adding a caption indicating who is shown in the photo is a big help!)
There is plenty of room for all the photos you can share!

Upload multiple photos at a time by clicking on
+ Add Your Photos
or by dragging and dropping photos from your file system onto this page. Once your photos have been selected click
Start upload
on to begin the upload or
to cancel.

Pictures that show up sideways need to be rotated offline and need to be uploaded again.