Abstract: The present invention relates to an accessory device (20) for an image-capturing device (10) for capturing a skin image of a subject's skin comprising a tubular member (21) having a first opening (22) at its first end (23) configured to surround a light source (11) and an image sensor (12) included in the image-capturing device (10) and a second opening (24) at its second end (25) configured to surround a skin portion (101) and two mirrors (26, 27) arranged within the tubular number (21) at its second end (25), wherein a first mirror (26) is arranged to retied light (30) emitted by the light source (11) towards the skin portion (101) and a second mirror (27) is arranged to reflect light (31) reflected from the skin portion (101) towards the image sensor (12). The accessory device can be coupled to an image-capturing device to acquire an image of a skin portion, which is evaluated to determine a skin parameter, such as skin gloss.