Image names are formatted differently depending on the setup of the site. Listed below are the different setups you can do for the site and the naming convention that it will follow.
Standard Setup - This uses the ProductID for the name.
Default image - productID.jpg
Multiple images - productID_#_.jpg
Multiple images with size/colors - productID_#_colorname.jpg
Using the image filename override feature - Images must be placed in their specific folders namely: icon, medium, and large folders which can be found under {root}/images/product.
Default image - filename.jpg
Multiple images - productID_#_.jpg
Multiple images with size/colors - productID_#_colorname.jpg
Note: Even if you use the filename override feature, once you use the Multi-Image Manager, it switches to the standard naming convention.
Using the SKU for Image Name - This uses the product SKU for the image name (Setting: UseSKUForProductImageName)
Default image - SKU.jpg
Multiple images - SKU_#_.jpg
Multiple images with size/colors - SKU_#_colorname.jpg
NOTE: Only JPG, GIF, and PNG images are supported.