Photos may be uploaded by dragging and dropping them into the photo display area, or by using a file browser to select photos from your PC. On a listing that already has some photos, click on the green "+" button at the bottom right of your screen to open the photo uploading dialog again. To close the dialog, click the "X" in the upper right-hand corner of the screen.
New photos will be added to the end of the photo set. As soon as photos are uploaded and processed (indicated by the green light animation), they are saved and added to your listing in Bridge. You still need to click "Publish Listing" to have them sent to your MLS or broker system, however. By default, any new photos will be published to all MLSs and broker systems when you start the publish action.
Please see the Using Filters article to publish different photos to each MLS and broker system.