If you are using Photoshop there are several ways of doing it.
Using a brush, a seperate document or via a script are but a few.
A quick example is using a seperate file....
Create your logo/signature if transparency required save as TIF,PSD,PNG etc
Now open a document and start recording a new action.
File -Place (select your logo/signature) file
Double click to accept its position (this will be the center of the picture)
Ctrl/A to select the whole layer your logo is on.
Now if you want the logo/signature say bottom right but 5 pixels up..
Use your cursor keys press the up arrow 5 times and the left arrow 5 times.
Layer - Align layers to selection - Bottom Edges
Layer - Align layers to selection - Right Edges
Ctrl/D to deselect
Flatten if required
Do your save
Stop the recording.
You can now batch this action as per normal.
The problem that you will have is the size of document you will be applying it to, it will be small on a large document and large on a small document.
Scripting would be the way forward as files/text can be scaled.