A credit equals an optimized image?
1 credit = 1 image or thumbnail.
A credit is used each time ShortPixel 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.
More details about how ShortPixel credits work here: http://blog.shortpixel.com/how-credits-work-within-your-shortpixel-account/