I just started using GIMP which is more or less an Open Source version of Photoshop.

When trying to create an animated GIF i ended up spending quite some time Googling for answers, this is how I did it in the end:

  1. Create your banner images the way you normally would.
  2. Each frame in the animation needs to be it’s own layer. GIMP will use each layer as animation frames in the order they are listed. I created one Layer Group for each animation frame and named them Frame1, Frame2 etc.
  3. Add the number of milliseconds you want the frame to be shown to the layer name, e.g. “Frame1 (2000ms)” to show the frame for 2000 milliseconds (2 seconds) before showing the next frame. Make sure you write exactly as I have done, i.e. between parentheses and no space between the number and ms.
  4. Take a backup copy of your file (File – Save a Copy…) as the following steps will destroy your layers etc
  5. You cannot do the next step with Layer Groups so you need to make any groups single layers. Do like this for each Layer Group: Make the Layer Group visible (Eye icon should show). Right click the layer and select Merge visible layers. From the popup window select Clipped to image, check Merge within active group only and leave Discard invisible layers unchecked. Click Merge button.
  6. GIMP needs your image to be in Indexed Mode in order to save it as a GIF. From Image menu select Mode – Indexed. From the popup window leave Color map as Generate optimum palette and Dithering set to None. Click Convert button.
  7. Export as GIF using File – Export and in the Export Image Dialog make sure to both name the file so it has the .gif extension and also select GIF in the drop down. Click the Export button. From the Export Image as GIF dialog window check the As animation checkbox (and set any of the Animated GIF options you want). Click the Export button.
  8. Done!

Using GIMP 2.8.4 on Windows.