An image's caption is text that accompanies the image. The caption may include the title of the picture and/or an explanation of what is depicted in the image. Not every Wikipedia image needs a caption: some are simply decorative, while some others are self-explanatory. Captions should not repeat the article title name unnecessarily.
If you decide that an image does not need a caption, then please follow the advice at Wikipedia:Alternative text for images to specify appropriate "alt" text. Alt text should not be a mirror of the caption text. Captions can be added by using the frame or thumb parameter as part of an image tag. For example: [[File:imagename.jpg|thumb|Write the caption here]]
Read more:
Wikipedia:Manual of Style/Captions
