Set minimum size in megapixels for images uploaded on opencart admin product page

Our project is a multivendor print on demand website for artists and photographers created in opencart. Each artist has his own separate admin. For uploading and managing images we use the open source application from [login to view URL] (adjusted for opencart). To maintain our quality standard for printed images the uploaded images of the vendors (which are used for the prints) should be a minimum number of megapixels in size (important: megapixels = not megabytes). The calculation for megapixels ( 1000000 pixels) = pixels width of the image X pixels height of the image. For example an image with a height of 2000 pixels and a width of 3000 pixels has a total number of 6000000 pixels = 6 megapixels. What do we want to achieve? During the creation of a product in the opencart admin of the vendor the vendor needs to add an image. The image should be approved when the number of megapixels of the image = greater than or equal to to minimum number of megapixels set by our standards. If the number of megapixels is smaller than our standard the vendor should get an error message. VERY IMPORTANT: The calculation should only be done for images added on the admin product page. For all other parts like banners etc. we dont want this minimum number of megapixels restriction. Please write me a small message instead of a copy / pasted pompous one if you can help us. We like to speak with people and not robots :)