use images search on google. Click search, size and only look at icons. our trees are about 1/4 the size of icons. Look for images with just a few colors and hard edges. Picture of mickey mouse works, picture of your gray cat not. if your matrix is 20x50 you could try resizing as 20x20, 20x 30, 20x50, 50x50. remember the pictures effect allows you to move the image. sometimes seeing part of the image at higher resolution works.
if you have an animated gif you need something like http://ezgif.com/ to resize it.