i use the free website fotojet for all my recent thumbnails and i also used it for my banner too, the developer actually asked me to review it thats how i discovered it! it has a template option for youtube banners anyway so it is the perfect dimension if you want to use it, has pretty templates too (i used my own photograpy for my banner then just added stuff over the top, and i suggest for thumbnails using the same option, completely fill the screen with it, then add whatever edits you want. Then resize it to the size you need for thumbnails afterwards, its a really good little site! Just something to use if you want too for maybe future thumbnails