Q:

Link to large image version instead of original?

How can I best make WP link to the large version of the image in all posts (instead of the original one) without modifying all the data in the database? I'm probably looking for the best filter to attach to. 

A:

You can modify the content that is displayed on the site by hooking in to the the_content filter. However, this is where the hard work starts. You need to identify all the images, figure out the links to the large sizes, and replace them. This could be a costly operation, so you might want to save the post back to the database and set a (hidden) meta key to indicate you don't have to do it again next time.
Now, some hints on how to do this: to get info on the attached images, you use get_children() and specify you only want attachments: get_children($post->ID, array('post_type' => 'attachment')). Attachments are stored like posts, and you can request different sizes with wp_get_attachment_image_src() by passing the requested size ("large").

