Hello,
I understand the requirement of having a 500px (on the longer side) thumbnail along with the multi-resolution tiles. Unfortunately, the native krpano droplet options might not provide this feature directly. However, a workaround could be to use an external image processing tool or a scripting language like Python with an imaging library like Pillow to resize your panorama to the desired dimensions post tile generation. This way, you can automate the process and have a 500px thumbnail created alongside the tiles and preview image generated by krpano.
Best regards!