I'm not aware of any way to count the precise number of product images on a Shopify store. However, you can get a rough idea by multiplying the number of products by the average number of images per product.
Please note that monthly cost depends on how many image edits you require for that given month. For example, if you use our Minimize (compress) feature, the app will skip any images that are already sufficiently compressed and will not charge any credits to process them.
Also, if you pick a plan and later realize you need more credits, you can simply upgrade to a higher plan without being double charged. In this scenario, Shopify will bill you for the most expensive plan you were on during that month.