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:
add_action('admin_head','myplugin_add_media_popup_notice');
function myplugin_add_media_popup_notice() {
?>
<script>
jQuery(window).load(function() {
// the MEDIA buttons you want the notice on
jQuery('#insert-media-button').click(function(){
// 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
jQuery('body').addClass('addedMediaPopUp')
}
}
});
</script>
<style>
/* 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);
}
</style>
</style>
<?php
}