The most common cause for some cards to be missing their art is because you don't have all of the art wad files. There should be 27 art wad files. One should be "DATA_DLC_CW_ART_UPDATE.wad", and you should have 26 named "DATA_DLC_CW_ART_MAIN_1.wad" through "DATA_DLC_CW_ART_MAIN_26.wad".
As for using your own specified art for some cards, open up the deck builder. In the menu strip, go to "Tools>Setup Custom Data Folder". This will create a folder in your game folder which the game will read as if it was a wad file. If you place game assets in here, they'll load into the game, so put the images you want to use in "...\Magic 2014\DATA_DLC_DECK_BUILDER_CUSTOM\DATA_ALL_PLATFORMS\ART_ASSETS\ILLUSTRATIONS\".
Making the art to use is relatively simple. There's a guide on how to do so
here under Phase 2: Creating cards>Step 4: Modify and convert art. The basic idea is to get the image to 512x376 pixels, and then use Gibbed Tools to convert it to a .tdx image. There's a link to Gibbed Tools on that same page under Phase 1: Preparing the Environment>Step 2: Install all necessary tools (when you get to the forum post with Gibbed Tools, you need the binaries.)