07-27-2017, 08:44 AM
(This post was last modified: 01-13-2023, 09:01 AM by rich2005.
Edit Reason: typo
)
Ofnuts beat me to it but not going to waste my effort 
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Nice Banner.
The images you have are different sizes and different color modes.
The black can be removed using Colors menu -> Color-to-Alpha one layer at a time but there is a script that will apply that to all layers in one go.
I used some third-party scripts. Unzip the attachment and put the 3 scripts in your Gimp profile C:\Users\your-name\.gimp-2.8\scripts folder
The procedure is:
Open the snow animation
1. Change the color mode to RGB from Indexed Image -> Mode -> RGB
2. Apply the Image -> color-to-alpha-all-layers script to remove the black
3. Change the canvas size Image -> Canvas size to suit the banner (1004x204),. Use the all layers option as well.
4. Add the banner to the bottom of the layer stack File -> Open-as-layers. Move to the bottom of the stack.
5. Apply the Combine Background script: Filters -> Animation -> Combine Background.
6. Remove the bottom (original banner) layer. Not needed anymore.
Do any more editing while still in RGB mode before exporting as an animated gif.
There is an alternative. A script that generates snow and will create the layers for an animation. That is in the zip as well.
A video demo of both ways 4 mins
sorry, I must have deleted it in a 'tidy-up'
------------
Nice Banner.
The images you have are different sizes and different color modes.
The black can be removed using Colors menu -> Color-to-Alpha one layer at a time but there is a script that will apply that to all layers in one go.
I used some third-party scripts. Unzip the attachment and put the 3 scripts in your Gimp profile C:\Users\your-name\.gimp-2.8\scripts folder
The procedure is:
Open the snow animation
1. Change the color mode to RGB from Indexed Image -> Mode -> RGB
2. Apply the Image -> color-to-alpha-all-layers script to remove the black
3. Change the canvas size Image -> Canvas size to suit the banner (1004x204),. Use the all layers option as well.
4. Add the banner to the bottom of the layer stack File -> Open-as-layers. Move to the bottom of the stack.
5. Apply the Combine Background script: Filters -> Animation -> Combine Background.
6. Remove the bottom (original banner) layer. Not needed anymore.
Do any more editing while still in RGB mode before exporting as an animated gif.
There is an alternative. A script that generates snow and will create the layers for an animation. That is in the zip as well.
A video demo of both ways 4 mins
sorry, I must have deleted it in a 'tidy-up'