The yellow triangle warning on the top of right and side of the screen says your disk is almost full.
It is generally accepted that you need to keep between 15GB-20GBs of free space for your Mac to function correctly, your Mac is always writing and moving files around, so if you starve your Mac of space it could completely lock up. So move files you do not need on a daily basis from your Mac to an external drive, once you are satisfied they have copied over and are safe then you can delete them from your Mac. Only delete files from within your User folder structure, files that take up the most room are typically movies, images and music. If this makes any improvement on your display issues, great, although I am pretty sure the picture break up is an indicator that your graphics card is showing signs of failing and would need to be replaced.