How To Achieve Close Ups of Art Work

HI, I am selling my artwork on a website (Saatchiart.com - Kimberly Hubar) and would like to show close ups of my artwork to show the textural detail.  How do I make a close up and save it as a closeup and have the right resolution cropped out of my artwork to upload on the site? It has to be 1200 x 1500 pixels minimum.