Get An Image's Dimensions / Large Images Always Fail To Load

I have also Googled this issue plenty of times, but nothing seems to be working 100% and/or the "solution" found is several years old.I have a text field on a webpage where a user enters in a URL to an image and then clicks a button to preview it. I want to restrict the image's size, so if it is larger than 600x450 then it will be automatically resized to those dimensions.The problem I have with this code is that the myImage.onerror event always fires if the image is large. My question is: how do you make the script "wait" for the image to load completely before trying to retrieve the width and height? Or, even better, how do you prevent the image failing to load?

Also, I know how to get the image's dimensions via PHP; however, according to the getimagesize() function's documentation on php.net, the function downloads the image to the server before retrieving the image's width and height, which could be potential trouble if a malicious or ignorant user enters a URL to an excessively large image.