<tml:image derivate ="derivate-query">
The attribute contains a derivate query which OpenWGA will use to search for a derivate of the adressed image that matches the query. If the query succeeds in finding a matching derivate it is displayed instead of the original image. If not the request will return with HTTP error code 412 "Precondition failed".
Note that the query is not executed on rendering the <tml:image> tag but on the request by which the browser will load the image.
This loads an derivate of the addressed image which is of type "poster" (i.e. a variant of visual representation) and should be about 1024 pixels wide:
<tml:image file="photo.jpg" derivate="usage=poster,width~1024"/>