This macro is just a shortcut for embedding an image and linking said image to its source.
{{thumbnail(url [, width])}} will embed the image at url into the document (wiki or issue) at size width, if specified. Width defaults to 90% and can be specified in either px or %.
The alias{{tb(url [, width])}} is a shorthand for the same macro.
The link has some default styling and class "redmine-thumbnaill-embed" for easily overriding these defaults.