Hi Tita,
40 images is quite a few.. especially if you change the slideshow size. Your server will be trying to crop/resize 40 images in one go. You would probably need to optimise your server if you're looking to use slideshows with that many slides in, at those sizes.
The images are only cropped/resized once, so as a workaround try uploading say 10 images at a time, then hitting save and preview, the moving on to the next 10 images.
Regards,
Tom.