@haurys - I have the same issue and was just about to ask the same question! I am currently getting around the default behavior by changing the "link to this post" option to "link manually" and then entering the full link to the post.
This pretty much creates the desired result (image on portfolio and/or home page slider links to post, but image on post no longer has the lightbox link to itself)... but it's not quite ideal because:
a) on individual post, I want NO link on the image ideally, and
b) it's a pain in the butt to have to enter every image's link manually and then worry about keeping them accurate if URLs change, etc.
Here's hoping someone can provide something we can add to functions.php or something which will remove the lightbox feature entirely from the individual post images while allowing us to use "link to this post" for the beneficial home page slider link, etc. There has to be some function kriesi wrote which adds the individual page's lightbox functionality, so hopefully there's something we can throw in functions.php which overrrides it.