First of all, it is a good idea to get your class familiar with different parts of a photograph. Words such as foreground, background, center, in the distance, left, right, top, bottom, and corners are essential for this.
Draw a rectangle on the board with some basic pictures and label these different areas. It is also handy to leave it on the board so they can use it for reference when doing their own picture descriptions.
Hopefully, your students are already quite competent with prepositions of place, but if they are not there are some suitable worksheets to practice this here.
Next, check the suggested vocabulary on the worksheets and make sure they know these words. Be sure to add more vocabulary where you see fit.
Finally, do a demonstration and describe a picture yourself for the class to see. This way they know clearly what their objectives are.
