When it comes to what kind of cover you want to put on your novel, there are multiple things to keep in mind. First, is genre, second is content, and third is market trends.
Genre plays a large roll in your cover. Readers should be able to tell what genre your novel is by a glance at your cover. A horror novel wouldn't have a couple embracing and a romance novel won't have a spaceship (unless of course it's sci-fi romance but even then, it needs more). Once you know your genre, the options for covers becomes more clear and less vast.
Content on your cover is as important as content within your novel. Every word on the cover has to pack a punch and every image needs to relay something. Having a picture of a random element from the story that doesn't matter too much to the plot will just clutter the graphic and make it confusing to the potential reader. Take this cover for example:
The couple kissing tells you romance, the castle and horse drawn carriage lets you know its a fantasy/fairy tale sub genre and the clouds and castle give it a darker feel even with lighter colors. The font is very fantasy. There is no doubt that this cover is firmly in the romance genre and even the sub genre shows through.
This cover is quite clearly not a romance. The dark figure with the gun, the big bold font, the scared woman, all point to action/murder/mystery.
Finally we need to discuss market trends. Trends are constantly shifting, but typically one element stays on for a while. Sometimes its a font (Pristina ring a bell?) sometimes it's an image type (half naked couple embracing or muscular man chest for example), but over all, covers go through trends just like popular novels. If you plan on releasing relatively close to purchasing your covers, current trends are great and a good artist can give you a cover to fit the trend while still having yours stand out from the crowd. If you don't plan to release for a few months, trust your designer to be ahead of the pack and be the one to set the trend. Your sales will thank you.