In the name of each image file, the part before the dot should be the id of the object you want this to become the image of. Neither the capitalization nor the file type matter. The upload may take some time, if your zip file is big. When it finishes, you will be returned to the Controls page. If the upload succeeded, you should see a new image on the page of each object for which you uploaded an image.
Some browsers impose a lower limit.
If uploading a very large zip file doesn't work, try a smaller one. All rights reserved.