Captions to tables, images and figures give the reader context for what they are looking at. Consequently, it is important to provide a good description of each table, image and figure in your text.
Steps
Part 1 of 2: Write the caption

Step 1. Be descriptive
The first rule is the most important. Tell the reader exactly what happens in the image. Why this one? The reader should be able to respond once the caption has been read.
For example, if you include a photo of a field on text that talks about biology, the caption should specify the importance of the field in the discussion

Step 2. If you are describing a table or graph, talk about the variables
What do the sides of the graph represent? The reader should have enough information from the caption, legend and graphic to make it understandable regardless of the text.

Step 3. Save the humor for other moments
Unless you are writing a comic text, the captions tend to be serious, due to the need for synthesis.

Step 4. Be brief
It shouldn't be more than a paragraph, but really, one sentence should be more than enough. In fact, complete sentences shouldn't even be needed. For a photo, a very short sentence, like “Martha on the boat”, is fine.

Step 5. Remove all unnecessary
For example, the caption above may have said "Martha greets from the huge giant boat", but the additional information is not necessary to make the reader understand what happens in the photo.
Part 2 of 2: Include the sources

Step 1. Mention the source below the graph or below the table if it comes from somewhere else
How you do it depends on your stylistic choices. Below, you will find how to provide sources in various formats.

Step 2. Cite in the style of the Modern Language Association
For example: "from Bob Davis, Motorcycles on the Road, (Boulder: Mountain Road Books, 2004) 55. Print."
Note: the caption starts with “from”

Step 3. Quote in the style of the American Psychologists Association
For example: “Note. (caption). Reprinted from Motorcycles on the Road (p. 55), Bob Davis, 2004, Boulder: Mountain Road Books. Copyright 2004: University Press. Authorization granted."

Step 4. Quote in the style of Chicago
For example: "Source: Bob Davis, Motorcycles on the Road, Boulder: Mountain Road Publishers, 2004, 55."