Well, if you need a program similar to photoshop, I recommend a program called Gimp. It's what I use for all my graphics.
I assume you are talking about thumbnails for your videos? If so, the easiest way to make them is to look up wallpaper for the game you are playing and use that as the background. Then all that is left is to add text on top of that, preferably using a custom readable font.
They don't have to be fancy, they just need to be ... not ugly. XD If that makes any sense. Some games you will benefit from taking snapshots and using those as the backgrounds.
If you have more questions, just ask.