Even with the previous post included I had the problem that the previous and next buttons were not showing up. Therefore, when having the same problem, try this:

In product_info.php and in /includes/modules/additionnal_images.php:

Change

rel="lightbox"

to

rel="lightbox[product]"

In lightbox.css

Change:

#prevLink:hover, #prevLink:visited:hover { background: url(../images/prevlabel.gif) left 15% no-repeat; }
#nextLink:hover, #nextLink:visited:hover { background: url(../images/nextlabel.gif) right 15% no-repeat; }

to

#prevLink:hover, #prevLink:visited:hover { background: url(./images/prevlabel.gif) left 15% no-repeat; }
#nextLink:hover, #nextLink:visited:hover { background: url(./images/nextlabel.gif) right 15% no-repeat; }

Its just a matter of a wrong image include. If even that does not work just create a direct link to the prevlabel.gif and nextlabel.gif (eg. #nextLink:hover, #nextLink:visited:hover { background: url(http://www.yourdomain.com/catalog/images/nextlabel.gif) right 15% no-repeat; }.

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