This problem deals with an unfortunate discrepancy between file extensions .jpg and .jpeg -- The results equate to frustrating 404 errors and lightbox plugins that seem to be broken.
Here's what happens:
1) Upload an image with file extension '.jpeg' (note the 'e') and insert it into a post, with a blank 'Link URL'.
2) Now that the image is in the body of the post, edit the image's Link URL. (hover-over the image in the post and click the small icon on the upper-left that appears). If you click the 'File URL' button to auto-full the Link URL box, it will autofill the correct image address, but with a '.jpg' extension!
This lead to 404 errors, a lightbox plugin that appeared broken, and had me thinking it was an .htaccess issue from my hosting company....
NOTE: it autofills with the correct file extension when clicking the 'File URL' button is clicked upon initially inserting the image, however it fails when using the secondary menu that is available after inserting the image.
Thanks for fixing this!