There seems to be some confusion and bad advice evident in this thread. As the developer of the Wilderness theme, please allow me to set things straight…
Firstly I don’t provide 24/7 support. If you contact me late at night I probably cannot reply until the following day. If an email lacks details of what you have read or tried already, then you’re likely to get a fairly vague response back from me, like links to existing support documents on the website. I think others on this forum may testify that I am one of the better developers, when it comes to customer support. I was still answering support emails on Christmas day!
Secondly my wiki, like all my websites, is copyright. I would appreciate if people would respect my work and refrain from copying content onto these forums - for the simple reason that when I update the wiki (which I often do), the old content will still be left on these forums. That poses the risk of misleading and confusing people who arrive at this thread at a later time. If you want to repost any of my content, the polite thing is to ask for my permission first and to correctly reference the source, including the link and date of retrieval.
The method of dragging and dropping a named image into page sidebars is a feature exclusively found in FreeStyle 2.0, in themes released after 28th November 2015. It is not supported in FreeStyle 1.0. The Wilderness RapidWeaver theme is still using FreeStyle 1.0, so that particular technique will not work for embedding banner images. I don’t know why it has been recommended on here - the themes that support FreeStyle 2.0 and named images are already clearly listed on the Wiki to limit confusion. Dropping images into a sidebar is not going to work unless the underlying framework of the theme supports it too. Expect this to be something added in Wilderness 5.2.
If you want a copy of the Wilderness theme (or any one of my themes) that has FreeStyle 2.0, then you simply have to ask me for it via email. That will then give you the capability to drag and drop named images into your sidebar. I have not had time to release many theme updates in recent weeks, owing to illness, all the other RapidWeaver announcements (see the forums for details) and the fact I’m awaiting the arrival of Bootstrap 4. It would be easier for all involved to release one single theme update amalgamating all changes, rather than multiple updates in close succession.
The example demo site for Wilderness can be downloaded using the link in the product page sidebar. That will demonstrate exactly how the banners work and give you an instant project file to build actual websites with. These project files are free to download and I do not charge for them or impose any other restrictions. The Wilderness demo site is making use of the FreeStyle code snippets, within the sidebars. These short snippets pull an image from the theme and embed it safely within the designated theme banner. The image will scale responsively and the addition of ALT attributes will greatly improve SEO - of particular importance to content placed above the page fold. Over time, FreeStyle has proven to be one of the very best opensource theme plugins for embedding banner images.
Regarding theme updates, all my RapidWeaver themes have support for Waterfall update checking (see the Nimblehost website for details). I was one of the first developers to adopt this update checking system into my themes. We NEVER destructively overwrite themes - instead each version of the theme is installed into RapidWeaver alongside older copies of the same theme. That’s why when you use my themes, you see the version number printed next to the theme name. I’ve never heard of one of my theme customers installing an update and complaining their modifications or custom images have been trashed! Of course if you plan on using a theme with different projects or making major changes to it; the wise thing to do would be to duplicate the theme and give it a new name. Duplication and renaming will omit the theme from update checking; so that theme becomes orphaned and is yours to do what you want with.
As part of the theme purchase, I am happy to duplicate / rename the theme and include any number of custom images you wish. I get asked to do it all the time, nearly daily. This is already mentioned on the ThemeFlood website. I know if it not a service very many other RapidWeaver theme developers are willing to provide, but it is something that exists here. I can show you exactly how to incorporate custom banner images and check they are suitably sized for what you want to accomplish.
Resources (previously known as assets) is a feature built into RapidWeaver and is not specific to any single theme. I would think Realmac Software has articles about resources on their support website or an official video tutorial. There is no PDF user guide for RapidWeaver 6, but you can save any page from the support website as a PDF, if you need to access it offline. Resources are basically custom files hard-linked to a project, which you can embed into pages using a macro. Resources can be safely used in banners, ExtraContent containers and just about anywhere else in a project. Personally I would not recommend using resources as a first choice for embedding images into a theme banners - there are better and easier ways of doing it.