Well, I don't know if this is what you want, but you can open hypergraph, choose the camera tab, right-click the camera that has the projected image you want to delete, then choose Graph>Show Upstream and Downstream. In the bottom section of hypergraph it should now show you the camera and the image. Select the image and delete it.

As far as the polygonal plane technique goes, it's real simple. You create a plane, scale it however you want, right-click it, choose materials>assign new material>lambert. The attribute window pops up. To the right of the color tab is a B&W box. Click it. Select the "File" texture in the window that pops up, then press the folder button to browse for the image you want to put on the plane. Make sure you display the textures by pressing 6 and you're done.