Open Textual's preferences, and under the style section click "Browse style files" to start editing your textual theme. Add something like the following below. It's just CSS so you can do just about anything. There's also a JS API you can tie into.

gistfile1.txt

/* Apply a background image to both the #lullabot and #drupalizeme channels */
body[channelname="#lullabot"] {
	background: url(https://dl.dropboxusercontent.com/u/63787/lullalogo.png) no-repeat fixed bottom right;
}

body[channelname="#drupalizeme"] {
	background: url(https://dl.dropboxusercontent.com/u/63787/dmelogo.png) no-repeat fixed center center;
}