I don’t want to edit core files in order to change the page where a user enters new posts, so is there a way to do it within a theme, perhaps within functions.php ..?
Specifically, I’m trying to change the wording of the ‘Set featured image’ text to something like ‘Set featured image – 50 pixels by 50 pixels’
You can use the filter
admin_post_thumbnail_html
:One way is to use careful jQuery code inserted in the header of the admin to modify the text within a specific part of the page. You need to be able to target a specific div/span/html tag in order to do so, but this system will let you edit any part of the page even if there is no filter (though the answer by Sorich has a great filter for the use-case in your question, my answer applies more generally to your question itself).
I used the code below to alter the featured image box in 2.9 to say ‘featured image’ instead of ‘post thumbnail’ because the labels changed in 3.0 and I wanted my old site to use the new terminology before upgrading it.
Note that the code handles both the popup box and the metabox, and uses some jquery AJAX functions to ensure the content gets re-modded after pageloads in the iframe.
Easier solution here:
The main idea is: re-declaring the meta-box with the required title.
Replace the post-type for which you want to edit the default “Featured Image” label.