During testing, we need to use released prometheus image, but currently the prometheus image stream uses the same registry as all other syndesis stuff:

name: '{{.Syndesis.Spec.Registry}}/{{.Images.PrometheusImagePrefix}}/{{ .Images.Support.Prometheus }}:{{ .Syndesis.Spec.Components.Prometheus.Tag }}'

Removing .Syndesis.Spec.Registry and prepending PrometheusImagePrefix in config.yaml with docker registry should help (the same as with oauth proxy):

Images:
  OAuthProxyImagePrefix: quay.io/openshift
  PrometheusImagePrefix: <add registry here as in quay>prom