Using AppleScript to find information about images in Pages documents
Nice work you almost got it. I would suggest looking at the properties instead of the data ;) 

tell application "Pages"
    tell front document
        repeat with pg in pages
            tell pg
                set im to images
                repeat with i in im
                    tell i
                        get properties
                    end tell
                end repeat
            end tell
        end repeat
    end tell
end tell