admin notice on Insert Media popup screen

I’m unaware of any hooks that tap in in the pop up, but there might be.

If not, you could use some jQuery to put your message on the pop-up when Add Media is clicked. I was able to with the following:


function myplugin_add_media_popup_notice() {
    jQuery(window).load(function() {

        // the MEDIA buttons you want the notice on
            // give the lightbox half a second to add itself
            setTimeout(myplugin_add_media_popup_notice, 500);

        function myplugin_add_media_popup_notice() {

            // if we haven't added a notice already
            if (!jQuery('body').hasClass('addedMediaPopUp')) {

                // the the notice
                jQuery('.media-frame-router').before('<div class="update-nag notice"><strong>Note: my message </strong></div>');    

                // and mark it to not show again if media btn reclicked
        /* and some css since .notice isn't completely defined at this scope */
        .media-frame .update-nag.notice {
            position: absolute;
            top: -10px;
            right: 40px;
            z-index: 999;
            width: calc(100% - 600px);
            border-left: 3px solid #168fc0;
            box-shadow: 0 0 4px rgba(0, 0, 0, 0.3);