Do a Google search for "iWeb 3 themes" and you'll find a number of possible candidates. However, the best way to create a unique website, IMO, is to start with a blank page in either the White, Black or Modern Frame theme. Those three allow users maximum modification/design opportunities. The Formal theme is another theme except for the blog and podcast pages. All layouts except for those two can be modified.
If you want to add an animated gif to an iWeb page you just drag it onto the the page as you would an image file. Flash can be added via HTML snippets. Roddy's iWeb for Musicians site has several pages on adding flash audio and video players:
Flash files are easy to incorporate in a webiste since all you need is to reference the file in an iframe code. Example here...
Remember that flash is falling in popularity due to the fact that mobile devices - in the main - can't play flash files.