Glitch effect won't work I'm learning After Effects from scratch so sorry if this is obvious but whenever I try to make one part of an image 'glitch' I create a section of the image and place it in the middle but when I press play my desired glitch (JPG noise) does appear but black pixels appear outside the selected area of the image, anyone got any idea how I can stop this??

Thank you
I think you should use the "Bad TV" plugin to achieve this effect.  It has a setting to glitch images, and make them look as though the encoding has broken:

http://aescripts.com/bad-tv/

An example: