First you need to find were that user downloaded her art, download that and put it in the C:\programs\WoW\Interface\addons\discordart\Custom textures
Then (add them to the folder while the game is offline) launch the game and enter the world, hide the minimap border with a suiting addon (dfm, visor) and launch dart
at the select texture part, you type add custom texture and type in the texture name BUT EVERY SINGLE CaPITAL LETTER MATTERS, just that non capital would have messed it up, so don't forget to check the spelling
That ought to do the trick fot you.