How to change this picture ?

Hello,
Each time we quote there is a small picture displaying inside the quote field. The problem is, this picture looks bad with my current forum theme.



I wanted to change it but I can't edit templates due to the version of my forum, and in the pictures management part (advanced mode), almost everything is blank (I installed the theme I'm using through Hitskins). By then, how can I change it? Maybe to start you could tell me what is the normal field to change it in the pictures management part?

Thank you in advance,
Vangerer

Using phpBB3?
Go to display >> Colors >> CSS stylesheet
Find this part in CSS and remove the photo link ( https://2img.net/i/fa/prosilver/quote.gif ) or replace it with an other photo.

Code:

blockquote {
    background: url("http://2img.net/i/fa/prosilver/quote.gif") no-repeat scroll 6px 8px #EBEADD;
    border: 1px solid #DBDBCE;
    font-size: 0.95em;
    margin: 1em 1px 1em 25px;
    overflow: hidden;
    padding: 5px;
}

Hey!

There wasn't this part in my CSS, but I did paste yours nonetheless and changed it to fit my forum's layout. And it worked! Surely not really "clean" way to do it, but it works.
Thank you very much!