Report - image - custom action - button

Report - image - custom action - button

I'm looking for info or codes to integrate an image on a "custom action" button in a report and not just text. Is it possible? 

In the report 
custom action "Etapes"
(
  workflow = Etapes
show action in view header = true
show action in edit record menu = true
column header = "Etapes"


The URL workflow : 

openUrl("https://xxx.zohocreatorportal.eu" + "#Report:Etapes_Reportwf?Fiche_compl_te_PSTwf.ID=" + input.ID,"parent window");