OS X Lion - Please help me understand Finder "Find" I'm a long-time Mac user, but for some reason I can't get my mind around this. Maybe I'm missing something incredibly obvious.

Baseline: When I choose Finder->File->Find, I get a window titled 'Searching "MacHD"' where "MacHD" is one of two Search places listed just below, to the right of the text "Search:" I see two potential search-places currently defined, "MacHD" and "This Mac". "MacHD" is the default, which generally works for me -- except when I need to restrict the search.

Q1:  How do I add to the list of potential search-places?  Say, I often want to search ~/Documents/Foo only.  To the right, I see a Save button and a "-", but, intuitively, I kind of expect a "+" button there.  I assume there's some kind of equivalent, but I can't see it.

Q2: What are the parameters of this Find? How would I find out? System Preferences -> Spotlight -> Search Results allows me to re-order and eliminate some search results from visibility, and System Preferences -> Spotlight -> Privacy lets me excludes some places from indexing, but how do I know, for example, if a file inside the System folder is or isn't findable?  Yeah, I can do a trial search and find that the file /System/Library/Accessibility/AccessibilityDefinitions.plist is NOT found, so I infer that Spotlight doesn't index the System folder. But is there a more direct way of figuring this out?  Could I add the System Folder to Spotlight's indexing?

At the moment, I'm searching for a mislaid image file among tens of thousands I've collected in various places on my HD. My procedure: I set the Finder find to look at "MacHD" and tell it to find all image files, change the results window to icon view, use the slider at lower right to make the icons "large", grab a cup of coffee, and scroll through all the images. But I'm not confident I'm getting a look at all the images I've placed throughout my HD. Is there a better, more thorough search method? Can I get more control?

Yes I know about the shell (command line) find command.  In one command I can get a list of, say, all *.jpg files on my HDD, but I can't see a convenient way of displaying and scrolling through them that's nearly as good as the Finder provides.

No doubt there's a tutorial for using Finder "Find" somewhere on the web that will put me right, but I haven't located one.
You have a few options since you're on Lion. The Find feature of Finder by default will search whatever folder you're in. The option next to it is called "This Mac" which should look through the computer. You can also go to "All My Files" in the sidebar and then click on the images section header and that should look for all the image files as well.