Metuchen High School: GIMP
The third link in the Gimp Links section below has a few examples of animated GIFs created with the Blend tool.
The first step in creating an animated GIF with Blend is to locate or create several images that are related to each other. (I used 4 images of a tree in each of the seasons.) You put each image on a separate layer of an image. The layers will blend into each other in the animated GIF: as one fades out, the next one fades in.
When you choose the Blend option (Filters > Animation > Blend), GIMP adds additional frames that transition between the layers in your image. The number you enter for Intermediate Frames determines how many frames are added between each layer. A greater number of layers leads to greater smoothness in the resulting GIF but increases the file size.
Here are the 4 images I began with:
This is the complete GIF:
I found it necessary, after using the Blend tool and creating the intermediate frames, to manually enter the time delay for each layer.
To create the animated GIF, do File > Save. Then do File > Export. Hit the plus sign next to Select File Type, in the lower-left hand corner of the dialog. Choose GIF image and click Export. Click the As Animation box. You can select Loop Forever. For "Frame disposal where unspecified," choose One Frame Per Layer. Click "Use disposal entered above for all frames". Click Export. Then preview your animated GIF: Filters > Animation > Playback.