Begin with a general statement to catch the reader's attention. This introductory sentence needs to contain a piece of intriguing information about the painting or artist and then conveyed in a relatively short number of words. It also has to hook the reader into finishing the paragraph that will most likely be summary in nature.
Make an outline of the descriptive facts about the painting that you wish to convey. Your list might look like this:
1. medium
2. date of completion
3. art period
4. subject matter
5. historical note and/or conclusion
Find out when the artwork was completed and include this pertinent piece of information. When writing about historical art, this little tidbit is critical, for it helps the writer tie the artwork to a specific era.
Describe the medium as best you can. If the artist has supplied any information, be sure to include it. Medium is a widespread topic area, but historical fine art paintings were either done in oil, egg tempera or frescoes. Drawings might be done in charcoal, lead, watercolor or ink, plus printmaking techniques include woodcut, intaglio, aquatint and wood engravings. This info should be put into one concise sentence.
Explain the subject matter to the reader. Many times this can be done in one succinct sentence, but a second sentence can be used if necessary. This is the heart of your paragraph, for this part of the analysis could be of the most interest to the reader.
Wrap it all up with a concluding sentence. There are probably a few more things that need to be conveyed in the writing. The piece of artwork might be part of a series, inspired by unusual events or part of a theme that includes many pieces of art. Let the reader know about this.