An image with nonempty alt text, occurring by itself in a paragraph, will be rendered as a figure with a caption. The image's alt text will be used as the caption.
![This is the caption](/url/of/image.png)
How this is rendered depends on the output format. Some output formats (e.g. RTF) do not yet support figures. In those formats, you'll just get an image in a paragraph by itself, with no caption.
If you just want a regular inline image, just make sure it is not the only thing in the paragraph. One way to do this is to insert a nonbreaking space after the image:
![This image won't be a figure](/url/of/image.png)\
Note that in reveal.js slide shows, an image in a paragraph by itself that has the stretch
class will fill the screen, and the caption and figure tags will be omitted.