Hiyah! I'll try and explain it as clear as possible.
If theres something you don't understand simply ask. ^w^
Example;
If you wanted to make a bloody rock texture for lets say the normal Feralheart rocks, you'd go into where the rock texture is located.
*Feralheart>Media>Objects>Rocks. Pick the texture you want.
*Open it up in your prefered art program. (Photoshop, Sai, Gimp)
*Add the blood splatter to your liking(You could download blood splotch brushes to make life easier, but you can just use the normal bush tool.)
*Save it as something different. Like name it "Bloodyrocks"
*Open up the rock.Material file in the same place as where you found the rock texture with notepad or what ever you feel comfortable with.
*Copy and paste the example that Enoki gave you into that Material file, waay at the bottom.
*Replace rockTex8.jpg with your Bloodyrocks.jpg (or png if thats the type you set it to,)
*Replace rockTex8/Material with what ever you think is easier to remember.
*Open Fh and go into the object maker and select a default FH rock.
*Now type what you replaced rockTex8/Material with into the box that says; "Material" Under Mesh tab.
And there you go. Brand new texture. You can do this with just about any object. Like trees, Walls. Ect.
Hope this helps!