Add dust and scratches to a photo in GIMP (free)

Open the dust plate as a layer, set the mode to Screen and lower the opacity. Invert it and use Multiply for dark dust.

GIMP is free and can do everything you need to add dust and scratches to a photo. The trick is the same as in Photoshop: a dust plate on pure black, set to Screen.

Step by step

1. Download a dust plate, for example slide scan dust.
2. Open your photo in GIMP (File → Open).
3. Choose File → Open as Layers and pick the dust JPG.
4. In the Layers panel, open the Mode list and choose Screen. The black disappears.
5. Lower the Opacity slider to 50–80%.
6. If the plate is not the same size as your photo, use Layer → Scale Layer, or the Unified Transform tool (Shift+T).
7. Save the result with File → Export As and choose JPG.

Dark dust instead of white

Select the dust layer and choose Colors → Invert. The specks turn black on white. Set the mode to Multiply. This looks like dust on a print rather than on a negative.

Do not repeat yourself

If you edit a whole series, flip or rotate the dust layer for each photo (Layer → Transform → Flip Horizontally). The same speck in the same place on every image gives the trick away.