Angular-js default image

I have the following code:

<img ng-if="weathermap[itemValue('BOM_Precis_2')]!=null"
						src="{{ServerPath}}/images/{{IconSet}}/{{weathermap[itemValue('BOM_Precis_2')].replace(' ','-')  | lowercase }}.png" />

id like to default the image to

"{{ServerPath}}/images/{{IconSet}}/cloudy.png" 

if the first image doesnt exist - I cant for the life of me work out how to construct this -can anyone help?