When I wanted to save it as gif however, it failed. It seems Krita in Ubuntu doesn't have support for GIFs because Krita uses graphicsmagick, a fork of imagemagick that won't be included in Ubuntu anytime soon. Free software is great, but it has its problems too... The solution was to use ImageMacgick's convert. Did you know it's as easy as
convert image.gif image.png
As Krita doesn't have support for animated GIFs, I've had to use the gimp anyway. And I again encountered a problem with the gimp: to be able to specify the delay between 2 frames of the animated GIF, I had first to do a "save as". A simple save didn't do the trick.
Anyway, now the Profoss finally has usable banners. Take a look!