Open the image you want to use as an alpha channel (or that it contains the alpha channel you want to use) in gimp.First let's copy the alpha channel to be used later with the following steps: The guide assumes the alpha channel is to be found as a component of another image. In any case, whichever method you used, the result should look like this (actual transparent PNG on left, with checkered background on answer is correct but assumes a little bit of gimp knowledge, so I am writing a step by step guide here, so it's easier to follow for people with zero gimp knowledge. Then optionally apply the mask as in method 1. on the base image layer and choose "Selection" from the dialog. Method 3: As method 2, but after Channel to Selection, don't invert the selection, but simply use Layer → Mask → Add Layer Mask. Add an alpha channel for it with Layers → Transparency → Add Alpha Channel if it doesn't have one already, and then cut the selection away from the layer ( Select → Cut or Ctrl+ X). Select the base image layer in the Layers dialog. Invert the selection with Select → Invert or Ctrl+ I. With the grayscale image layer visible, open the Channels dialog, right-click one of the Red/Green/Blue channels (they should all be identical) and select Channel to Selection. Method 2: Open the images as layers ( File → Open as Layers.) or just paste one as a new layer into the other. Optionally, turn the layer mask into an actual alpha channel using Layer → Mask → Apply Layer Mask (but simply exporting the image as PNG will do that for you). Paste the grayscale image in ( Edit → Paste or Ctrl+ V) and anchor the resulting floating selection into the mask ( Layers → Anchor Layer or Ctrl+ H, or click the Anchor icon in the Layers dialog). The layer mask you just added should be selected for editing if not, select it with Layer → Mask → Edit Layer Mask (or just click the mask in the Layers dialog). It doesn't matter what you choose as the mask, since we'll be replacing it shortly. (also available by right-clicking the layer in the Layers dialog). On the base image, select Layer → Mask → Add Layer Mask. Select the grayscale image and copy it to the clipboard ( Edit → Copy or Ctrl+ C). There are several ways to do this let me describe a few of them.
#Picture fades into another picture shapes license
(Base image based on this photo by John Fielding, used under the CC-By-SA 2.0 license alpha mask created by me using the GIMP's Sphere Designer tool.)
OK, so I assume you have two images: a normal image that you want to add transparency to, and a grayscale image that you want to use as its alpha (transparency) channel, something like these:
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