Bob click on Edit. Then more options then click on upload a file. Then you can chose the pictures. If they are screenshots they will be located in %appdata% then .Minecraft then the screeshots folder. It might be easier to access them if you drag the photo you want to upload onto your desktop or a folder there.
I looked at your plot. It's a nice design but it is not a build. Look at other designer plots that won and see what we are looking for.
