There are no "Photos" in Aperture. What you see are Images, often called "Versions".
The most important thing to immediately learn is that Images are not files.
To have files to share with other programs, you must first create them. You create them by exporting from Aperture. You specify the file format, and the particulars within the format you select, in an Image Export Preset. So you need to select the Images you want to upload, export them from Aperture using whatever Image Export Preset you set up (or using one of the supplied ones), and then, from Finder, select these exported _files_ and drag them into the uploader box.
Some photo hosting sites provide one-dialog export-and-upload (viz.: Smugmug, Zenfolio).
Message was edited by: Kirby Krieger -- typos. Been AFK too long.
Only if you want to export the Masters of your Images. "Masters" are the files you imported. You almost certainly are trying to export your Versions. "Versions" include any changes you've made (adjustments and/or metadata).
You can access the Image Export Presets from that dialog (at the bottom of the drop-down select "Edit") or via "Aperture➞Presets➞Image Export ... ".