The error reported by the SDTT is only used to get the result rich in Google articles. If you do not want to get a rich result, you can ignore the error.
To get the rich result in Google articles, the value for
picture must be either a URL or a Schema.org
So, you can either withdraw the FOAF
Picture type, so that you only provide a URL value:
Or you can provide a Schema.org
ImageObject + FOAF
(You can ignore the fact that Google's SDTT does not know about FOAF.
Picture type; the tool usually only knows Schema.org, but it's fine to use other vocabularies.)