This is going to be my first Inkscape tutorial so please forgive me for any mistakes I might make and please send your comments to notify me of anything I might have missed during these steps. If you ever wanted to make a cool looking wallpaper using Inkscape, then this is a good start. I absolutely love the Warhammer 40k universe and so decided to make a wallpaper using artwork from the 40K universe. So lets begine. First of all I would like you to download the image below so that you can use it in the tutorial. Click on it to enlarge it and then save the image on your computer. 

After this you can fire up Inkscape, select New and then select a Desktop 1024X768 option for the canvas. Next up we select the File>Import dialog box and browse for the image to import inside Inkscape. 

Use these settings after selecting the image and then clicking on path>trace bitmap. Use the update button to see how the changes have come along. If you are not satisfied with the way the image has been traced, change the settings in the brightness cutoff by using the slider buttons and then click update again. Once satisfied, click OK.

You should come up with the following result. You can now hide the original picture which is underneath the traced out one by Alt+Clicking to select it. You can delete the original picture by pressing the "delete" button on your keyboard since we don't require it through the rest of the tutorial.

Place it in the middle of the canvas as shown. Note the brown rectangle represents the canvas borders