Beej

In WordPress, attachments default to having their own pages. Sometimes, 
this is not desirable, as in search engine results. Use this small plugin to 
redirect your attachment pages to the post parent using a 301 redirect 
(post parents are also searched for by post name). If an attachment parent post 
cannot be found, it redirects to your blog home page with a 302 redirect.

Attachment Page Redirect X

Redirect attachment pages to their parent page (301), when possible, otherwise to the blog home page (302).

LoudSkyMedia

This simple plugin permanently redirects all 404’s to the main blog URL. There may be several reasons why you might want to do this, but the primary purpose is to siphon Google Page Rank (TM) from missing pages to the homepage.

404 Redirection

Permanently redirect all 404's to the main blog URL. The primary purpose is to salvage Google PageRank (TM) from missing pages.

Attachment Page Redirect X vs 404 Redirection