Create a watermark with GIMP
> Layer> insert new layer
Create a watermark-text with the text tool
Set the transparency (opacity) of the layer in the layer window
Unite the layer a level down
Export the image in the format you want
> Layer> insert new layer
Create a watermark-text with the text tool
Set the transparency (opacity) of the layer in the layer window
Unite the layer a level down
Export the image in the format you want