Use custom button ( picture ) for contact form

Hello there,

i want to use a picture ( which looks like a button ) as the “submit”-button for the contact form.
( For one special contact form, not for all contact forms on my website )

I am used to the quick css field in the general styling tab and i already enabled custom css.

I already used some picture as buttons on my website because i wanted to have a gradient background.
Maybe there is a similar solution like for the custom bullet points.

Thank you in advance