Expecting the answer to be using jsInterop but never hurts to ask.
Can the following be done in Blazor:
const img = new Image();
img.onload = function() {
alert(this.width + 'x' + this.height);
}
img.src = 'http://www.google.com/intl/en_ALL/images/logo.gif';
As seen here:How to get image size (height & width) using JavaScript?