What do you mean by an img "placeholder"?
I have asked users to shoot for sizing their images before uploading them to the server for the page to grab and display within a certain range to avoid images being subject to pixelation and so on. The problem is they are still uploading pictures that are more panoramic while others are very much profile pics. So text is moving around on the screen. So it was my hope I could specify a limitation for their images so they wouldn't get beyond a certain width and height requiring me to resize them down taking the largest of those two numbers (width or height) and sizing that parameter down to some maximum. But in order to do that I need some way of knowing what the original pics dimensions are so that I can programmatically choose which parameter to set.
The img tag looks like:

Images are being hidden/displayed based on clicking a next/previous button so "style" is set to none when a user clicks back/forward past the currently displayed image. The onerror just displays a standard "missing photo" picture instead of image.jpg if it's not found.
But there is text below the image that gets changed along with the picture when the user clicks on the next/previous buttons. And it's annoying to see the buttons and text below the picture move up and down the screen because images being displayed has such different heights. It's even a greater problem to set the height leaving the width to dynamically adjust causing text and other content displayed on the screen to move around horizontally. So between the two options, setting the width is much better.
I was just hoping to eliminate the movement issue altogether by being able to force all images to stay within the same size/boundaries on the screen.
No way to do that?