The minimum requirement is 500 pixels on the AU site as well, with a recommended size of 1600 pixels (on the longest side), and most images will get a little orange flag on them saying something to the effect of not being to the optimum size if they are under that, but they should still upload and display without a problem - if they're not, it may be a TL issue (I think there have been a few of those lately), but I can only suggest contacting eBay if the images are being prevented from uploading.