This started to become annoying. Although GIMP has some very nice features and can be considered a real candidate to Photoshop, some of them can be quite hard and counter-intuitive to use. One of them would be the ability to paste a layer inside a layer mask - or in other words use another layer as a layer mask.

Why would anyone want to do this? I love textures - for me they bring a 3rd dimension to photographs, which makes them seem more alive. They can make cold photos seem more warm, they can also add an emotional side. There are lots of textures types and so are the modalities of using them in photos.

One way would be to copy the main layer of the image, change the curves/levels/hue or anything else and add a layer mask to this layer. The layer mask will now decide what's visible and what's not - and this can be the perfect place to place the texture.

But GIMP doesn't have a very straightforward way of doing this. Not that it would hard, tedious or anything like that - it's just not very suggestive. Here are the steps for using another layer as a layer mask:

1. First, select the layer that you want to use as a layer mask (Select->All)
2. Copy this layer to the clipboard (Edit -> Copy)
3. Select the layer where you want to place the layer mask. Add a new layer mask (Layer -> Mask -> Add new mask) - and make sure the layer mask is selected (it should have a white border around it)
4. Paste the previously copied layer (Edit -> Paste). Don't worry, this will appear as a "Floating Selection" which is perfectly normal.
5. Now go to the Layer menu and select "Anchor Layer". If everything went fine, you should have your copied layer placed inside the layer mask.

Note: Although these steps focus on using an existing layer as a layer mask, the procedure is quite similar for using another image.