The syntax used for displaying an image is:

[[File:{name}|{type}|{location}|{size}|{alt=}|{caption}]]

Only [[File:{name}]] parameter is required.

Do not put spaces between parameters. The other parameters are optional and can be placed in any order. Some infoboxes do not require the brackets. Keep parameters in lower case. The other parameters are:

Type

'thumb' / 'thumbnail' or 'frame'. This causes image to be displayed with specific formatting. "thumb" is normally preferred.

Location

'right', 'left', 'center' or 'none'. Determines placement of the image on the page. "Left" or "right" is the norm, but large panoramas or timelines can be displayed in the center.

Size

{width}px or {width}x{height}px (e.g. 50x40px, would limit width to 50 pixels and height to 40 pixels). Normally only one variable is used. Use common sense when determining the sizes; you can use the "Show preview" button if you need to. If thumb or thumbnail is chosen, size should normally be left out, so that the size defaults to the size set in a user's preferences.

alt=

(keep it lower case). This is the "alternate image" parameter used to describe the image for screenreaders or for people with low-vision. It should be more descriptive than the caption alone. Do not use this for another copy of the caption or of the article title, as the reader will already be aware of these.

Caption

Any element which cannot be identified as one of the above is automatically treated as caption text. It is traditional to put this last. The caption should identify what the image is, and ideally be a complete sentence that adds to the article by pointing out something a casual reader would not have noticed otherwise, or add information the pertains to the image. Full sentence or multi fragment captions require full stop punctuation.

If you have created a picture that is not already in Wikipedia's image collection on the Commons that you want to include in an article, you will need to upload it first. Bonus tip: Similar formatting is used to insert basic audio or basic video clips into articles.