Take the SD card from your 3DS and put it into your computer, there should be a labeled slot for it to go into, a pop-up should pop up. Go into the SD cards library of images and search for the one you want. When you find it, click and drag the image into your computers image library. Once you have that, get onto TBT and make a post in which you want your picture. Press the little photo tree button in the hot-bar, click search files and find your picture. Ta-Da! You got your picture! I hope this makes sense!