You can't use a Substitute= statement to modify a URL used for downloading an image. The Substitute happens "after", not "before" the RegExp is executed. Before you think of it, as I certainly did, you can't just use a subsequent WebParser measure using the result of the first one to do the downloading. WebParser measures, like all plugins, are not "dynamic" in that way, and the substitute is ignored by the second WebParser measure using the first as the URL=. Any way you slice it, you are going to get the image reference created by the RegExp.
There may be some clever solution involving Lua, but due to the fact that plugins are not dynamic, even then the best you can probably hope for is that you can get the image, but it will mean doing a refresh of the skin on each execution of the WebParser measure, which is ugly with hair on it.
