You can copy and paste. eg. make a selection of an image you want to use as a texture and Ctrl-C (or Edit - Copy). Go back to your image you want to apply the lighting effects texture on. Click on Channels tab, click on Create New Channel icon at bottom of palette, Ctrl-V (or Edit - Paste). Click back on the Layers tab and select the layer you want to apply the effect too. When you go to Filter - Render - Lighting Effects, select Alpha 1 (or whatever you renamed it to) from the Texture Channel at bottom of the Lighting Effect dialog box and click OK.