Juicebox has not actually had an update in many years so, if you have previously used Juicebox with acceptable results, I'm not sure what might have changed.

Your gallery images look OK to me but, of course, I do not know how sharp you intent or expect them to be.

It sounds like you have already tried changing the resize settings within JuiceboxBuilder-Pro.
The first image in your gallery is 1000 x 1500 which is probably a fine resolution for web gallery usage (for most devices, depending on your target audience). Just make sure that the 'Quality' is high (default is 80, maximum is 99).

If you like, you could skip the resizing entirely and just have Juicebox display your source images by deselecting the 'Resize Images' checkbox on the 'Images' panel. (You might need to resize your source images manually before feeding them to JuiceboxBuilder-Pro if they are very large.)
Additionally, you could prevent Juicebox from dynamically resizing the images (and just display them in the gallery at actual size) by setting imageScaleMode="NONE" in JuiceboxBuilder-Pro's 'Customize -> Main Image' section (although please be aware that cropping may occur).

I hope these suggestions help.