You might try customizing one or both of these image-dired options (the text is what you get with C-h v).
image-dired-thumb-relief - Size of button-like border around thumbnails.
image-dired-thumb-margin - Size of the margin around thumbnails. This is where you see the cursor.
There are many other image-dired options, some of which might also help you more ...
I am not a fan on my approach, but I ended up redefining (org-fontify-meta-lines-and-blocks-1).
I am posting the idea here :
(defun org-fontify-meta-lines-and-blocks-1 (limit)
"Fontify #+ lines and blocks."
(let ((case-fold-search t))
"^\\([ \t]*#\\(\\(\\+[a-zA-Z]+:?\\| \\|$\\)\\(_\\([a-zA-Z]+\\)\\)?\\)[ \t]*\\(\\([^ \...