Hi. I had the same problem, but i made it work by changing the flickrViewer.php file.
It generates an incorrect xml file, because it writes backgroundimagePath instead of backgroundImagePath (the I in image should be uppercase for the xml to work correctly)
In flickrViewer.php, in the /*Generate the XML OUT*/ you should see something like this:

$xmlout = '<?xm' . 'l version="1.0" encoding="UTF-8"?>';
$xmlout .= '<!-- Last updated: ' . date("r") . ' -->';
$xmlout .= '<simpleviewerGallery 
maximageHeight="' . $maximageHeight . '"
maximageWidth="' . $maximageWidth . '"
textColor="' . $textColor . '"
frameColor="' . $frameColor . '"
frameWidth="' . $frameWidth . '"
stagePadding="' . $stagePadding . '"
thumbnailColumns="' . $thumbnailColumns . '"
thumbnailRows="' . $thumbnailRows . '"
navPosition="' . $navPosition . '"
enableRightClickOpen="' . $enableRightClickOpen . '"
backgroundimagePath="' . $backgroundimagePath . '"
title="' . $title . '"
imagePath="flickrViewer.php?mode=img&amp;size=large&amp;image="
thumbPath="flickrViewer.php?mode=img&amp;size=thumb&amp;image=">
';

in that code you have to change the following line:

backgroundimagePath="' . $backgroundimagePath . '"

to

backgroundImagePath="' . $backgroundimagePath . '"

Just that, changing to uppercase should make it work.
hope this helps