artemisia gentileschis judith decapitating holofernes is part of what story
The Rokeby Venus (1647-51) by Velazquez.
National Gallery, London.
For analysis and explanation of other important pictures from the Renaissance, see: Famous Paintings Analyzed (1250-1800).
Judith beheading Holofernes was a very popular story amongst Baroque artists. Artemisia Gentileschi’s contemporary Johann Liss stayed abreast with the Baroque style by including macabre imagery in his painting, Judith in the Tent of Holofernes. The painting shows the headless body of Holofernes slumping over. Judith sweeps Holofernes’s head into a basket showing a look of swiftness about her. The viewer can see the maidservant’s head in the background while the rest of her body is unseen. She seems eager to see what directions Judith will give her next.  The decapitated body of Holofernes has blood gushing out of it, showing Liss’s interest in the human body.  Gentileschi has a similar urgency in her painting but shows Judith in mid-decapitation rather than showing Holofernes headless body. Gentileschi also uses the same amount of bloodiness in her painting. 
There have been many different interpretations and viewpoints on Judith Slaying Holofernes by art historians and biographers alike. Art historian Mary Garrard believes that Judith Slaying Holofernes portrays Judith as a “socially liberated woman who punishes masculine wrongdoing”.  Although the painting depicts a scene from the Bible, art historians have suggested that Gentileschi drew herself as Judith and her mentor Agostino Tassi, who was tried for and convicted of her rape, as Holofernes. Gentileschi’s biographer Mary Garrard famously proposed an autobiographical reading of the painting, stating that it functions as “a cathartic expression of the artist’s private, and perhaps repressed, rage”.  Griselda Pollock suggests that the painting should be “read less in terms of its overt references to Artemisia’s experience than as an encoding of the artist’s sublimated responses to events in her life and the historical context in which she worked.”  More recent discussion of the painting has moved away from too close a relationship to the rape of Gentileschi; rather it has focussed on Gentileschi’s determination to paint strong women who are the centre of the action. 
Self Portrait as Saint Catherine of Alexandria, oil on canvas, by Artemisia Gentileschi (about 1615-1617). Courtesy of Wikipedia.
The bed is stained red. Holofernes struggles in vain against the attack. The parabolic trajectory of the blood leaving his neck was likely influenced by Gentileschi’s friend, mathematician and astronomer, Galileo Galilei. Judith Slaying Holofernes was initially deemed too gruesome to exhibit.