It turns out that although you can't do it with CMD-C and CMD-V, you can drag and drop the thumbnail of a photo from iPhoto into Facebook. So I did it this way:
Find a photo I want to post
Press the space bar to go back to the thumbnail view. The thumbnail of the photo I was looking at is highlighted in yellow.
Click-and-drag the highlighted thumbnail and, while holding the mouse button down, press CMD-tab to switch to the browser
Drop the photo into Facebook
Repeat for other photos until the post is finished
It's not exactly ideal but it did work. I won't accept my own answer in case someone has a better way.
iOS native photos app doesn't support dropbox sharing although you can open dropbox app in you device and add files manually. Make sure you have given dropbox permission to access photos.
Details:
Manual Upload
Step 1
Launch the Dropbox app on your iPad, and sign in to your Dropbox account if prompted.
Manual Upload
Step 2
Navigate to the folder on your Dropbox account to which you'd like to upload a file. If you'd like to create a new folder for your files, tap the plus sign at the top of the left column and then tap "Create Folder." Enter the name of the folder, and tap the "Create" button to create your new folder.
Step 3
Tap the plus sign at the top of the left column, and then tap the "Upload Here" button.
Step 4
Tap all of the files you'd like to upload to your Dropbox, as you can upload multiple files at once. A blue check appears in the upper right corner of the files you've selected.
Step 5
Tap the "Upload" button to upload the files to your Dropbox folder.
Automatic Upload with Camera Upload
Step 1
Launch Dropbox on your iPad, and log in to your Dropbox account if prompted.
Step 2
Tap the gear in the lower right corner of the left column to visit your Dropbox settings.
Step 3
Tap the "Camera Upload" tab, and then switch the button from "Off" to "On." Your photos and videos automatically upload to Dropbox from your iPad's storage.
source
Best Answer
You can't import photos directly from Facebook in Keynote for iOS, but you can add photos from your Camera Roll. And the Facebook app for iOS lets you save photos to your Camera Roll -- open the photo in Facebook, then select 'Save Photo' from the top right menu.
You have to repeat this process for each photo you want to include in the presentation, but it's the only way I can think of.