So, to make your life simpler...use the shorthand background property. (Link for more details about it)

If that site's unclear or you're confused by the other parts, this is what you need to add:
background: url("DirectImgLink") Color no-repeat bottom right;

Only change these two bits:
DirectImgLink (leave the quotes around it) - This is...a direct link to your background image. A direct link means one that will end in a file name (ex: .jpg, .png, .gif).
Color - This is jsut whatever color you want the rest of your background to be set as. You can use an HTML color name, a hex, or you can use RGB if you'll probably want to use a hex, though. (If you don't want to set a color, then simply take that part out.)

This is the selector for your comment background (you need to add the above code into the declaration block for it and remove what's currently there): .comments li[data-user="Pixdeen"] blockquote

(Right now the declaration block for your comment background is a long, wild mess because you have a gradient background, but just make sure you everything inside the brackets.)

If that's confusing, it will look like this:
.comments li[data-user="Pixdeen"] blockquote {
background: url("DirectImgLink") Color no-repeat bottom right;

Whatever you do, make sure you're not adding code, but rather are replacing the same code that's already there.

You may also want to change the color on the little triangle piece of the message bubble (legit don't know what to fucking call that rn, lol).

That's this:
.comments li[data-user="Pixdeen"] blockquote:after {
border-color: transparent COLOR COLOR transparent !important;

The reason it's like that and has two colors is because you're overriding another code and that code is an "effect" of sorts to make the triangle shape appear. You'd replace both of the "COLOR" parts with the same color (or you could use two different ones, but it would look weird) and leave the rest alone.
(The "!important" bit if necessary for it to override wikia's stylesheet.)

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