Using the defualt new tab feature that isnt possible without a addon.

• addons.mozilla.org

I don't mean the "new tab" grids, i'm talking about opening a new tab while browsing with the "old tab"

What is the new tab URL (browser.newtab.url)?

It is probably possible to set a background image via code in userContent.css
This is an example for the about:home page, place the image in the chrome folder along with the userContent.css file.

Add code to the userContent.css file.

• http://kb.mozillazine.org/userContent.css

The customization files userChrome.css (user interface) and userContent.css (websites) are located in the chrome folder in the Firefox profile folder.

• http://kb.mozillazine.org/Editing_configuration

/* about:home */
@-moz-document url(about:home){
 body{ background: url("abouthome.jpg") rgb(180, 218, 244) center no-repeat !important; }
 #snippetContainer { display: none !important; } /* hide the snippet container */
}