In principle, you shouldn't have this problem.
Try to see if you have these lines (1405 line and so on):
http://github.com/balupton/jquery-lightbox/blob/master/scripts/jquery.lightbox.js
if ( this.auto_resize !== false )
{ // We want to auto resize
var maxWidth = Math.floor(wWidth*(4/5));
var maxHeight = Math.floor(wHeight*(4/5));
var resizeRatio;
while ( iWidth > maxWidth || iHeight > maxHeight )
{ // We need to resize
if ( iWidth > maxWidth )
{ // Resize width, then height proportionally
resizeRatio = maxWidth/iWidth;
iWidth = maxWidth;
iHeight = Math.floor(iHeight*resizeRatio);
}
if ( iHeight > maxHeight )
{ // Resize height, then width proportionally
resizeRatio = maxHeight/iHeight;
iHeight = maxHeight;
iWidth = Math.floor(iWidth*resizeRatio);
}
}
}
Here you have iWidth which is the image width and maxWidth which is the width of the screen.
If the iWidth is bigger, it equals to maxWidth.
It must work.