well, this MAY help:
In the admin settings locate the title style section. Check "Enable shortcodes within title".
Then, when specifying your title, you can use other shortcodes but note you must not reuse [,]. Instead, use (,) or pick your own open/close characters. In this manner, you can use any wordpress shortcode to snaz-up your spoiler title.
HTML *may* work: (img)image.png(/img). Im unable to test this for you at the moment, sorry.