Hmmmm...
I am sorry to partly disagree:
I've checked the permissions of the images folder in my OSX 10.7.5
and tough the picture itself has correct rights, the folder above (images & thumbs) is too restricted, thus the forbidden error (and not a pure 404)
-rw-r--r-- 1 alvaroprudente staff 133012 9 Nov 19:36 PB060032.JPG
-rw-r--r-- 1 alvaroprudente staff 85982 9 Nov 19:36 PB050015.JPG
-rw-r--r-- 1 alvaroprudente staff 90287 9 Nov 19:36 PB050006.JPG
-rw-r--r-- 1 alvaroprudente staff 82231 9 Nov 19:36 PB050002.JPG
Manzanita15:~ alvaroprudente$ ls -ltr /Users/alvaroprudente/Sites/CaneloDigital/PaginaNueva/simpleViewer/
total 32
drwxr-xr-x 7 alvaroprudente staff 238 9 Nov 19:35 svcore
drwx------ 52 alvaroprudente staff 1768 9 Nov 19:36 thumbs
drwx------ 52 alvaroprudente staff 1768 9 Nov 19:36 images
-rw-r--r-- 1 alvaroprudente staff 587 9 Nov 19:38 index.html
Thus the error:
Uder unix/linux/OSX and apache not only the specific rights of a file do have to fit but also the whole permission structure above: folder, overfolder, etc.
I've checked the URL you suggested gives this error:
Forbidden
You don't have permission to access /juicebox/images/this_image_does_not_exist.jpg on this server.
Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.
So you see in first instance you get the forbidden error due to "no read rights" on the images folder.
Then in second instance you get the 404...
But anyway to correct this you simply have to change the rights of the images & thumbs folder to 755 and voila...