I was messing around in photoshop making a gimp care bare (as you can see in the background of my photoshop. LAWL.) and I came across a nice way to make a blood background maybe for a signature or avatar or what not and I'll teach you how to make it!
Step 1: New Document
Okay, you're going to start off by creating a new 5 inch by 5 inch document.
Step 2: Fill It In
Alright, go ahead and take your paint bucket tool select black or blackish color and fill in the new document/layer with black.
Step 3: Render Fibers
The picture is selecting clouds but what you're actually going to do is select fibers instead of clouds.
Step 4: You've Got Fibers!
Now your picture should look something like what I have, you should leave the defaults for a look like i have or you can play with it, it really doesn't matter it will still look like our base idea.
Step 5: Your Difference Clouds!
Now go ahead and go back to filter then render then difference clouds, you can do this once or for a better effect do it twice.
Step 6: The Finale!
Okay now your going to change the hue and saturation, but don't fret I have the settings for that right here, or you can play around with it a little and create your own little effect.
Once you're done with that you can add text or some wierd little picture because I know how freaky all you instructatards are ; P just joking! But yep thats the basis of it all and it's really simple and easy to understand!