Sometimes a painting will require a long description. This depends on the context of the painting. If it is not described in the surrounding text, and it is important to the reader to know the details of the image, then you can write a long description.
The following example is from DAISY's webinars on Long Descriptions and where written by Huw Alexander from textBOX.
[Alt-Text] An oil painting by Pieter Breughel the Younger entitled A Village festival in Honor of Saint Hubert and Saint Anthony.
[Long Description] The scene is one of mass revelry along the main thoroughfare of a village in celebration of the religious festival of the Saints Anthony and Hubert. The street is filled with revelers and events. Brueghel has contrasted the indulgence and misbehavior of some of the villagers with moments of happiness and care from others. The scene is very detailed and is divided into 4 quadrants to aid the visualization of the scene.
Upper right quadrant. The section includes the following 10 events.