I think that the concepts are nice, but there might be a few improvements.
First off, the black is really overwhelming. I kind of wish that it wasn't quite so dominant. This might be alleviated to a degree by making the graphics larger.
What also might help is giving the trees and standing figure much more contrast. For one thing, they disappear almost completely even at the fairly large size I am seeing them here on my home screen. It's hard to imagine them holding up on a smaller screen or at thumbnail size. (And for another thing, it does seem to be a little odd to have "White Tree Chronicles" as the series title and represent this with a dark grey tree.)
Finally, the covers really aren't terribly informative. It's not too hard to tell that the novels are probably fantasy...but beyond that generalization they really don't say much at all about the books specifically.
Skoob_ym is right in saying that "the cover doesn't give much of a clue. I am having to rely on the words to know what to expect. Still, it could work."
In order for someone to read the back cover blurb, they are going to want to be interested enough to get that far. I just don't think that these covers would draw that much attention...especially, as I said, most of the artwork being so murky and low in contrast (in fact, I had no idea that there was a small figure on the back cover of "Oval" until I blew the cover up to nearly full-screen size). At thumbnail size, the grey imagery disappears almost entirely.
I am afraid these are really not very much better. The backgrounds come off as not much more than dark, amorphous blobs. At thumbnail size, there is essentially nothing there at all (see above). Even the logo comes off as grey and you even begin to lose readability in the type because of its relatively small size within the cover.
You are headed in the right direction, however, in making the imagery fill the front cover: you just need to make the background images readable at a glance...which is all that a book cover may ever get. A cover needs to attract attention, not be a puzzle for the potential reader to figure out. It is one thing to intrigue, quite another to have someone say, "Huh?"
It's perfectly OK to be different...but it shouldn't be done just for its own sake. The difference needs to serve a purpose. Your book is going to be seen in the midst of hundreds of others and while the nearly solid black covers might draw attention for a moment I think that attention will be fleeting since once they have caught the eye...there is nothing else.
If your books are in fact fantasy adventures, the covers need to reflect this quality which, I am afraid, they are not doing.
I agree with Skoob.
I have no doubt but that you like the "simple look," but I think that the old bugaboo of subjectivity is getting in the way, which is a stumbling block for a great many authors creating their own covers. The cover works for you because you are intimately familiar with the book.
There are a couple of problems with going the route you have chosen. The first has already been addressed several times and that is the lack of information the cover provides. I realize that you try to deal with this by adding a tagline...but a potential reader is going to have to want to stop and read that...and I don't know if this cover is compelling enough to make that happen. Too, the tagline will disappear at anything like thumbnail size. Besides, a cover should not depend on a written explanation to succeed.
Another problem is the layout and design itself. The text floats around in an overwhelming sea of black, with very little tying any of the graphic elements together. Each block of type---title, series title and author name---has almost equal emphasis. This not only dilutes any visual impact the blocks of text might have, the strong vertical symmetry makes the overall effect of the cover one that is extremely static.
Finally, the cover is relentlessly monochromatic.
Any "punch" that the stark black might have created is negated by the symmetrical, static design.
The net result of all of this is a cover that suggests very little about the nature, subject or themes of your book, with the starkly static, monochromatic design doing very little to convey any sense of fantasy or adventure.
For instance, the graphic device you use on each cover---the tree and sword, the tree and the decorative ring---are all well and good, but they do not suggest a story. For example, simply putting a sword on the cover of a novel---and displayed with such perfect, static symmetry---really doesn't say a great deal, other than that perhaps somewhere in the course of the novel a sword appears. That just isn't enough. The ring is even less engaging and informative (largely since much of its significance requires prior knowledge of the story).
It is perfectly possible to create a design that is graphically very simple (and with only spots of blue and black very nearly monochromatic) but energetic at the same time and suggestive of a story...
Or take this case of this cover. It, too, is a simple image on a flat black background. The image of the briefcase by itself might have been static, boring and uninformative...but the addition of the bullet holes and blood suggests not only the nature of the novel but that there is a story behind the cover.