Preferably JPG, but we can work with png. However, please stick with one format so that duplicates can be avoided. e.g. 123.jpg and 123.png
The images will be scaled to about 400x400, so images with a lot higher resolution may not improve the quality.
To associate the image with the product we will need the name of the file to correspond with the item number/SKU or UPC codes of the item. If the names are UPC or GTIN codes, they need to be a complete valid code with the system digit and the check digit.
If we are using anything similar to FTP, we can use that directly, or we can setup a Google Drive folder and add some users on your end with upload permissions to load the images in.