Paintings often offer a richer, more complete, and conventional visual cue than other art forms such as sculpture or contemporary video installation. This makes it easier to extract ideas from students and utilize vocabulary and speaking skills. After choosing a painting---say a full body portrait or a group scenario---think of some vocabulary words that would be useful in the interpretation or description of the painting. Let's say you choose a painting like the one here.
You could think of words that would describe the painting, like woman, mother, child, girl, happy, calm, love, etc., or if your students are more-advanced speakers, you would think of less common words related to the painting like cherubic (for the girl), glowing (for the skin), background, posture, composition, and so forth.
Once you have established the vocabulary you will use, activities can vary. The first step, no matter what activity you choose, is to engage students with the topic. Ask questions that will connect the students to the painting. For the painting above, for example, you might ask them if they've ever had family photos taken. (Most likely, all of them will have had one taken at some point.) Next, you can ask questions. For what occasion were the photos taken? Do you like how your family photos came out? Are you in the habit of collecting them? Why do you think people have family photos taken? Then, you could segue into the portrait, which is essentially the family photo of bygone times.
For a main activity, you could have students invent a personality or a life around one or all of the figures in the portrait; the specifics will depend on how many figures there are, how much time you have, and the students' skill level. If the portrait has a more-elaborate background, or there are two or more figures engaged in the action, students could be guided to create a story around the painting. Who are the people? What are they doing here? What are they feeling and thinking? What are they saying to each other? Are they in trouble/danger? If the painting is a single subject portrait, students can conduct a mock interview where they are given a set of questions and asked to provide answers for their subject. If the painting is a landscape, students can make travel brochures as if the image were a vacation destination. Or they can write a letter home describing the contents of the painting as if they lived inside of it.
Similar techniques can be applied to other visual mediums, like photography and sculpture. First, establish a useful vocabulary for your students to use around the subject, and then guide them through an activity in which they will be able to apply the vocabulary, connect to the work, and enjoy engaging in a discussion about it using the vocabulary you've given them.