I would also suggest that when you name your images do not leaves spaces in the name. For example:
The City Street shows as The%20City%20Street-Acrylic%20on%20wood.jpg
The %20 represents a space - if you must separate words use the underscore "_". And it's a good idea to always use lower case letters.
This image is 106 x 137 size and 86KB in size. I know nothing about lightboxes but a picture this size should be under 10KB.
Here's a good site for a free program to make
thumbnails.
Also, find a post here by Collectors-Info who has some great tips in his posts - Look
HERE