icons for customized buttons

[yoramdavid]

Hi, I am using XnView MP linux.
I would like to iconize some long text such as "Adjust image to window" but it is greyed out, is it not yet implemented?

I have created a command to an external app (Gimp), it works fine but when I want to select an icon, the icon chooser opens with the only format available being "ICON", I only know one icon format and that is .ico. Yet no icon appear in the icon chooser window, it does not see any image there.
I tried with other formats such as .jpg, .png, it does not see any.
What format does it needs to be? Why does it not sees an .ico?
Thank you.
David