ilamont: Thanks for posting that, it was interesting. I quite liked your first cover (though there are issues: background of the photo, was the use of the logo permitted, etc.), but agree that the professional job suits the work much better ... I'm trying to work out whether the fact that it reminded me of the "... for Dummies" series was a good thing or a bad thing.
But on the subject of professional cover designs: not all of them are so great. I see quite a lot come out that are so obviously cobbled together from clipart and seem to offer little in the way of making a book look appealing. This is true even (especially?) on books by big names published by big publishers.
I am not certain that there is anything anyone can do to make a book reliably stand out from a page full of small thumbnails - especially when matched up against books of the same genre (eg: your book up with "for Dummies" books, or a page full of erotica books where all the covers are half-naked men or women). But certainly the cover should look attractive on the larger thumbnail (when displayed on it's own or with a small selection of other books. Your blog links to a kindleboard page as an example of "pro covers", and while I agree that individually most look good, few really stand-out in their small thumbnail form.
I think this is a bit like something I said on another thread about the importance of the first pages in a novel: first do no harm ... and after that you can worry about doing good. I don't think your first cover did any/much harm, and it seems to me your sales reflect that.