I have been having fun installing galleries on some WordPress blogs this past week. I will admit NextGen Gallery is probably one of the more challenging plugins to install and work with- but very worthwhile. ( Not as bad as the Kimili Flash plugin tho!)
This was not a 30 second download the plugin and upload tho content folder operation. It took me easliy a half hor to get it up and workign properly and even then it is work to get each photo up.
Form and SEO point of veiw you need to make some adjustments to images once they are up.
For example in your NextGen galleries, change the alt text of the image. By default the alt text is name of the file so you alt text will be something like photo1.jpg. Change this to describe your photo is simple English.
A great tip is to make your images linkable.
How to Create Hyper Links For Images in Nex Gen Gallery Plugin for WordPress
This is quite easy although bit time consuming to do- but again very worth it.
When in Gallery there is empty field for description- you can add html here including a link. I will use this to link to a post that is based on the original image.