ShortPixel Image Optimizer - A credit equals an optimized image?
1 credit = 1 image or thumbnail.
A credit is used each time ShortPixel Image Optimizer optimizes an image or thumbnail by at least 5%. If we're not able to optimize an image or thumbnail by at least 5% then no credit will be used :-)
Please note that each image has a number of thumbnails automatically generated by WordPress upon adding a new image. The number of thumbnails depends on the theme and plugins you use. Each thumbnail optimization is counted as a separate credit.
You can also choose to exclude thumbnails that you don't want to have optimized thus reducing the number of credits needed.
More details about how ShortPixel credits work here: http://blog.shortpixel.com/how-credits-work-within-your-shortpixel-account/