Problem with Next Gen Gallery?

I had NextGen Gallery installed before I changed to the Atahulalpha theme when I de-activated Next Gen, then deleted it.

I found a plug in that allows users to upload images (NextGEN Public Uploader) so I installed that and then tried to re-install Next Gen gallery but it won't let me.

I tried through the wordpress interface (plugin install) then I tried just putting the files in the right place with Filezilla - the NextGen gallery shows up as in the plugin folder on Filezilla but not in the list of plugins on wordpress.

And...all the images that I had stored in the next gen gallery folders don't show up - the folders and images exist still but they dont show up

the error message I get when I try to install Next Gen is:

Destination folder already exists. /public_html/wp-content/plugins/nextgen-gallery/

Plugin install failed.

If I delete everything manually and then try to use the plugin install I get a different error.

I don't understand why it is there on Filezilla but I can't see it on the wordpress backend.

Any ideas much appreciated!
Lisa