If the images are going to keep their current name, then just specify the product ImageFileNameOverride in the excel file and manually upload each of the images to the images/product/icon, images/product/medium, and images/product/large directories making sure the name is the same as what you've specified in the ImageFileNameOverride column of the excel sheet. Another option, if you're interested in resizing the images, may be to use WSI. You can actually upload the images through WSI and configure it in such a way that image resizing will take care of the images before they are saved
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