Medieval art is two-dimensional and somewhat stylized. Greek and Russian Orthodox religious art, although from different eras, tend to follow similar stylistic conventions. Sources for angel designs of this type include illuminated manuscripts and Orthodox icons.
St. Michael conquering the dragon is a popular motif in this era, as are martial angels, clad in armor and bearing weapons. A design like this would be a good choice for a guardian angel tattoo.
Renaissance art codified the popular image of angels as androgynous, clad in flowing robes with halos over their heads. Artists to consider include Raphael, Albrecht Durer, Giovanni Francesco Barbieri and Gian Lorenzo Bernini. Tattoos based on this era should be fine black line or grayscale work.
If an angel tattoo memorializes a loved one, the angel designs found in cemeteries may prove inspiring. The body language of graveyard angels is quite expressive of grief, loss and mourning. Many angel statues have their heads bowed and their arms crossed. Combine this pose with some bold lettering for a powerful tribute to someone who has passed.
For a lighthearted take on angels, consider the classic cherub image, also known as cupids. Renaissance painters, such as Rosso Fiorentino and Raphael, popularized this playful angel with a child's body (known to art historians as a "putto"), and Valentine's Day cards have cemented its fame. Cherubs convey a lightheartedness and innocence of spirit when utilized in a tattoo design.
One of the most popular angel tattoo designs is a full back piece of feathered angel wings. This tattoo marks the bearer as an angel themselves, calling out their more spiritual nature or higher self or sometimes announcing a feeling of being blessed in life.
There is a variety of styles for angel wings on the back, ranging from a straightforward representation of feathers to a stylized tribal design.
The second main attribute that angels have, after wings, is the halo, a circle of light surrounding the head. A shining ring often represents a halo. Such a design, incorporated into a tattoo, would make a strong statement of commitment to a spiritual or moral path.
Contemporary fantasy artists have taken angel iconography to new heights. Whereas fine art angels are most often androgynous, fantasy art tends to depict angels as either wispily feminine or boldly masculine. These tattoos may feature fantasy outfits (or nudity), and swords and armor are once again common. Look to fantasy art for a tattoo design that feels modern and realistic.
An angel tattoo based on stained glass mosaics would be a striking and unusual choice.
Don't neglect iconoclastic religious artists, such as Marc Chagall and William Blake, for inspirational purposes.
Medieval Persian art has some fascinating, little-known angel images, often depicted in bold colors and sometimes with a double set of wings.