Imaging photoplethysmography (iPPG) has emerged as a contactless heart-rate monitoring technique. As the respiratory activity modulates the heart rate, we investigate the accuracy of iPPG in conveying the inter-beat variation due to the respiratory modulation of the heart rate. The instantaneous respiratory rate was estimated in real-time from the iPPG inter-beat variations with an algorithm based on a bank of short FIR notch filters. The comparison of the iPPG-based respiratory rate estimates to ECG-based estimates showed that the iPPG ones were only slightly less accurate in spite of the challenging conditions related to this contacless technique.