Another vote for Visio. Once you create an "object" such as an alternator, you can easily store it in a template file for later use. Software for schematic capture may be too specific and limited more to actual circuit schematics and can have features you don’t need (such as device models or checking a circuit schematic to a physical layout). For example, adding a component such as an alternator, solar panel or outboard to the drawing would likely be difficult with schematic capture software - but easy with Visio. I think Visio is fairly easy to use. I also don’t know what it costs but suspect its not all that cheap. It might come with Microsoft office now?
Once you’ve created a drawing, just save as jpeg for internet posting. You can easily do things like import pictures into your drawings.