If you need basic ideas and keywords, start with an encyclopedia or textbook first. Note down any useful leads, then use this for further research.
Image databases are also a good source of initial ideas and inspiration. You can browse images by location, time period, and type of art.
Use scholarly sources for your final bibliography and for in-depth analysis and insights.
The attached brainstorming worksheet may give you some ideas.
When analyzing a work of art, it's helpful to develop and apply a list of relevant questions. Consider:
Barnet's A Short Guide to Writing About Art provides many useful examples of analytical questions and is a required/recommended text for most Art History courses.