Adobe AIR (the platform on which JuiceboxBuilder-Pro runs) uses Adobe Flash Player as its runtime environment and the largest image size which Adobe Flash Player can handle is 16,777,215 pixels. This equates to a square image of 4095 x 4095. One dimension can be larger than 4095 but if the product of the width and height exceeds 16,777,215 pixels, then the error you have encountered will be triggered. Please see this web page for details: http://helpx.adobe.com/flash-player/kb/ … files.html
If you have larger images that you would like to use as source images for your gallery, you could perhaps resize them in an imaging program such as Adobe Photoshop before feeding them to JuiceboxBuilder-Pro. Rather than have your images resized twice (once by your imaging program and again by JuiceboxBuilder-Pro), you could use your imaging program to resize your images to the correct size for your gallery and then deselect the 'Resize Images' checkbox on the 'Images' tab (so that the images are not processed again by JuiceboxBuilder-Pro).
Alternatively, there are other methods of creating a Juicebox-Pro gallery which you might like to take a look at.
Please see here for details.