Upload iCloud photos with OS X 10.7
You should be able to do this at icloud.com. After you sign in, click photos, then upload at the top. You should then be able to select the photos you want to upload and click choose. Note all the photos must be individually selected: you may not select a folder.
You may alternatively drag them to the photos interface on icloud.com.

Obviously, it will take awhile for a few thousand to upload.