Those numeric suffixes are image sizes corresponding to the width/height values in wp-admin->Settings->Media. It is part of the thumbnail system. If the right image manipulation libraries are installed on the server, WordPress will create these different image sizes when you upload an image. The idea is that you can then load appropriately sized images for different locations and don’t have to load full size images all the time and scale them in the browser, which is not efficient.
I’ve used that importer (assuming you mean this one) and it did pull in those multiple image sizes, so I don’t know what the issue is with that.