To add new banners you should use the banner-template file. Just change the "Image" layer and export selecting just "Exporter" layer. Inkscape is a good tool to do that.

After that include your banner in this folder and name it with the last available number. Ex: 54.png, 55.png...

In the "basemod_synfig.css" change the line "$total = "2";" with the updated number of files of this folder.

Thats all!

Yaco