create a transparent document in Photoshop approx 600x400 pixels, then copy/paste your signature, or create a new one using the text tool. At this stage text size or colour is not important.
Now go to "Edit">>"Define Brush Set" Give the new brush a suitable name such as "My signature" or use the coyright symbol and save.
To use your "Signature" select the paint brush and from the drop down menu select the brush you have just made. Select a suitable colour and brush size and click in a suitable position on your photo.
You will probably need to flatten the text layer before selecting "Define Brush Set"