You got Photoshop? If not, get the GIMP.
Make a new image (256x256 pixels), with a transparent background. <-- important
Use the image tools to make a pretty spray on it.
Save it as a .png so you get your transparency.
Download a program called VTFEdit. Choose File -> Import. Select your png.
On the options that appear, most is probably alright already, but you should change maximum width and maximum height to 256x256. Texture type volume texture.
Click OK, then File -> Save As. Save it in Steam/steamapps/<your_steam_ID>/team fortress 2/tf/materials/VGUI/logos/
This is how I made mine. I just took a lame advert from the internet, added some grunge to it and tattered the edges.