2
you'll find the code to make the XOOPS textarea in /class/xoopsform/formdhtmltextarea.php.
In the render() function you'll find this text:
<img onclick='javascript:xoopsCodeImg("".$this->getName()."", "".htmlspecialchars(_ENTERIMGURL, ENT_QUOTES)."", "".htmlspecialchars(_ENTERIMGPOS, ENT_QUOTES)."", "".htmlspecialchars(_IMGPOSRORL, ENT_QUOTES)."", "".htmlspecialchars(_ERRORIMGPOS, ENT_QUOTES)."");' onmouseover='style.cursor="hand"' src='".XOOPS_URL."/images/imgsrc.gif' alt='imgsrc' />
Remove it and the "Enter Image Url" button goes away.