This tutorial shows how to add some realistic blood, just like this:
Step 1Open your image, I used this one:
To get it in full size, right click on it and choose "View image".
Select the Lasso Tool make a feather of 1px.
first, we'll add some dried out blood So make a selection of a bigger area than the one you want the dripping blood to be.
Right click on the image and choose Layer via copy.
You'll get a new layer with only the selected area.
Go to Image > adjustment > Variations.
Press More red and Darker few times until you get a nice color, since its just the dried out blood choose a light red.
Go to Filter > Blur > Guassian blur.
Choose a radious of about 2-3px.
Step 2Choose the Eraser tool
Use it to erase most of the blood, Set the opacity to 7%.
First use a relatively big brush (size about 25px) and stroke a bit around the edges.
Then take a small brush (3px) and stroke randomly to create some flow.
In the end result you should a thin layer of blood.
Step 3now, we'll add a little volume to the blood, since its not the flowing blood you might skip this step and it wouldn't affect much the end result.
Use the Burn Tool to darken some parts of the blood.
Set the exposure to 10% and use a brush size of 15-25px.
Next, select the Dodge Tool to lighten other parts a little.
Set the exposure to 10% and use a brush size of 2-3px.