Modifying the_content()

Hi guys,

I’m running a WP site, where every posts contains just an image. Then in the template I add next_post_link(‘%link’, ‘Next page’,TRUE, ‘7’); or previous link. But visitors are complaining about these link that it’s annoying to clicking them. So I’d like to modify the content in a way, that the image will link to the next post in the same category.

Any idea how to do that? I don’t want to change all the post (hundreds) manually in the editor.

Thanks a lot