GIMP/Caption a Picture

This effect will create a text caption with an outline effect. Firstly, use the text tool to write your caption. A contrasting caption would typically be black text with a white border; the border will be coloured later, so use black text.

On the text layer, use


 *   Layer > Discard Text Information
 *   Layer > Layer to Image Size
 *   Layer > Mask > Add Layer Mask

Choose "Transfer layer's alpha channel" as initial mask

Duplicate the text layer ( in Layers dialogue or   Layer > Duplicate Layer)

Select the lower layer's content, and fill it in white.

Switch to the layer's mask, and use   Filters > Generic > Erode, this should expand the lower layer and give the text a border. This may need to be repeated several times ( Ctrl+F )

Different fills and gradients can be applied to the two layers for a range of effects. Additionally, if a layer's mask is copied into a new, hidden layer, this can be used to apply a bump map via   Filters > Map > Bump Map.

A method that may be easier to follow is documented here with step-by-step photos.