Is it possible to run javascript before images begin loading?

Javascript loading and execution is serialized by the browser (unless you use something like head.js), but the problem is that the DOM has to be available for a script to modify it. The jQuery ready event fires after the DOM is available, so the browser has already started requesting the resources that were referenced in the HTML.

So if you put the Javascript before the image tag it won't be able to find the image tags, and once ready fires the download has already started. I'm not aware of any events that fire before image load (just one for aborts), so the cleanest method is to create the HTML with the modified src attributes in the first place.

Alternatively, put the src in a different attribute on the image (like data_orig_src) and run the script to set src to data_orig_src on each image upon document ready. Use CSS to hide the images before changing the src so the user doesn't see a broken image icon. I think this is probably better than adding the images after the fact because you won't need to track where the images need to be placed in the DOM, and it should perform better as well.

Of course if you can change the server to use data_orig_src instead of src, why not just put the proper src in the tag in the first place...

You cannot change the DOM of the page before the DOM has been loaded. And, once the DOM has been loaded, the browser has already started requesting images. So, you cannot change <img> tags before they start loading their images.

What you could do is change the source of the page to not have any of the images you want to change in the source of the page and then use javascript to dynamically insert the desired images after the page has been loaded. This way the browser will never request the wrong images.

Or, you could change the <img> tags to not have a .src property at all and with your Javascript you would add the .src property. An <img> tag with no .src property will not display until you provide it with a .src property.

If you're worried about the wrong images flashing as they are loaded before you change them to the correct images, you can use CSS in a stylesheet to hide the images initially and then after you change the img.src to the correct value and that new .src value has loaded, you can make them visible.

If you can't change the source of the page, then all you can do is hide the images initially (using CSS) until the correct .src has been set on them and that new .src value has been loaded.