I don't know of anything that does that. If the developer has been thoughtful enough to put all the images in the assets/images folder, you could look there, but you wouldn't know if you had all the images.
The only ways I can think of to reliably find all the images would be to
1. Get every page at the site with cURL and scrape it looking for image tags. That would give you each image's URL, and existing alt tag, if any.
2. Recursively walk the directory tree of MODX files looking for image files (.jpg, png, etc.). That would give you the filename and you could calculate the URL but you wouldn't know of any existing alt tags and you might miss some images. You'd want to skip the manager/ folder, which has many images that are used only in the Manager.
I would do number one. It would run much slower, but it would be more reliable, have more straightforward code, ignore Manager images, and give you more information.
That would look something like this:
$docIds = $modx->getChildIds(0);
foreach($docIds as $docId) {
$url = $modx->makeUrl($docId, "", "", "full");
/* Get page here with cURL */
/* Get Page base href with preg_match */
/* Get image tag data with preg_match_all */
/* add data to $output */
}
return $output;
If you have more than one front-end context, you'd need to do the loop above for each one.
