Go to your server and search for this file usually found in this path

concrete5_root/packages/jb_parallax/blocks/jb_parallax/view.php at line 160

change the value 750,750 to a dimension that you want to specify for example if you want it to be 1280 width by 720 height

echo "<a href='".$ih->outputThumbnail3($thumbf,1280,720,'')."' title="".$cleandesc."">";

The resizer is used mainly for the purpose of shrinking very large images like 1920 x 1080 dimension which when popped out it exceeds the viewport of the browser. As a result the lightbox will cover everything and forces the users who have smaller screen resolution (by average is 1280 x 720) to scroll in order to see the entire photo. However, if you don't want Parallax to resize it for you what you can do is modify the code (same line) just change it from this

echo "<a href='".$ih->outputThumbnail3($thumbf,750,750,'')."' title="".$cleandesc."">";

To this

echo "<a href='".$f->getRelativePath()."' title="".$cleandesc."">";