An image request is counted every time a new preview image is generated by the server. The images generated in your account are cached, this means that identical images reloaded multiple times are only counted once. The count is reset at the end of each month, you can view your usage status at any time under Menu -> Billing & Usage.
Please note that the cache is emptied every time the mats and mouldings are modified, this normally leads to a higher usage in the initial configuration phase.