Since I am redrawing the image again and again Nifty puts the new image again and again in the texture atlas until it is full. If I do not call markBatchRenderImageAsUnloaded() the image does not get an update. I checked the batch texture and it stores this image multiple times (once for each call of the methode).
So my final solution is a seperated Picture. I just attach it to the guiNode, no Nifty for this. This picute has the texture:
Picture p = new Picture("MiniMap_Picture");
p.setImage(this.desktopAssetManager, MinimapAppState.LandScapeTextureKey, false);
I am just updating the buffer of this image and store it in the assetmanager.