Oh, I found a description from The Encyclopedia of Tibetan Symbols and Motifs by Robert Beér, Page 314-315. I have highlighted the answer in bold. Please bear in mind that human skins are meant to be a symbolic offering and even those actual skins used are offered by people who have committed evil crimes and wish to atone their sins upon their death. Check it out!
Flayed Skin Friezes of Wrathful Offering Assemblies
The illustration on Plate 136 depict three drawings of the upper frieze or canopy of flayed human and animal skins that crown wall paintings or thangkas of the assembly of wrathful offerings (Tib. rgyan tshos). As thangkas the assembly of wrathful offerings are most commonly painted on a black background and often take the form of long horizontal banners, with red brocade borders on the upper three sides and hanging silks valances along the bottom. In this form they are hung upon, or represent, the walls of protective deity chapels (Tib. mgon khang), and are used in sacrificial ceremonies (Tib. bskang rdzas) to appease the wrathful deities; where the slain enemies are offered as a quivering mound of skin and flesh, representing desire and a glistening pile of broken and dry bones, representing hatred.
The central images of these black thangkas depict only the attributes of the deities and not their bodily forms. Several deities may be represented on a single horizontal painting, with only their mounts, hand-held implements, ornaments and attire depicted against the subtle shading of the deities' wisdom flames. A host of animal offerings - black horses, mules, yaks, buffaloes, dogs, goats, bears, camels, lions and tigers - with silk ribbons adorning their backs may stampede around the deities' invisible forms, showing that they have been ransomed from the butchers' knives and presented to the deities' as sacrificial offerings. At the sides are assemblies of weapons, and the musical instruments used during the ritual invocations to the protective deities. Across the top of the wrathful assembly of offerings - as an applique banner, painted thangka, or wall mural - is depicted the canopy of flayed human and animals skins. This canopy represents the upper wall and ceiling of the protector chapel, which is commonly hung with iron weapons, and stuffed animals and birds.