1) I am not sure I understand the question. If your website is running on Wordpress (a content management system CMS) you can use plugins built for that CMS. However if your website is running on a different CMS such as Joomla for example or Drupal, then you would need to use plugins built for those systems.
All websites are created via a process such as this - first laid out in Photoshop/Illustrator/Fireworks then are transferred to html/css. At that point the design is complete and a programmer creates a theme which includes the layout along with the database/interactivity/backend functionality using some programming language, which in the case of Wordpress is PHP and in addition uses syntax specific to Wordpress. A Wordpress theme without Wordpress is like a DVD without a DVD player.
I think your confusion can be cleared up by investing 3-4 hours watching basic youtube videos about Wordpress and CSS.
I suggest you watch these:
1) http://www.youtube.com/watch?v=it8jbSdderk (a bit longwinded, but he litterally doesn't leave any point unturned)
2) http://www.youtube.com/watch?v=nTkFxkjDvjM (can put an insomniac to sleep, but after 10 mins gets better)
The second video above, is all about the #1 tool you should get to know and love if you want to change your name from codeless to codechamp - firebug on firefox/or chrome developer tools on chrome. Begin with this 5min video http://vimeo.com/20917974
Regarding creating image galleries, please add the http://wordpress.org/extend/plugins/nextgen-gallery/ Nextgen Gallery plugin, which will let you easily create galleries anyway you want.