Dark Edges around imported PNGs ~

Hello,

I hope this is the appropriate place to ask this question. The PNGs I imported as planes all have a dark edge around them that behaves strangely. It has something to do with the alpha/transparency. Sometimes the edge will disappear when I select the object, sometimes it it disappears when I deselect. The edges are transparent and shimmery when I zoom in and rotate around them. They do not appear in the rendered image, but they are extremely annoying.

Please help!