There is that common saying, which goes, "A picture paints a thousand words."
This is true a lot of times, and words are also pictures. Since they are instantly recognizable, proper use can go a long way in conveying that message on the wall.
When I was in photojournalism college, my professors would always caution their students when they choose to include text in a photograph, because words command attention.
So, when you do include text, choose to 'go all the way'. Words can motivate. A client had requested a 20-inch by 30-inch canvas with only '200' written on it and nothing else. It would serve as a constant reminder to him.
There is a secondary beauty to text and that is in the font, and much has been written about it on the Internet.
How does a person prepare art/design canvas like the above examples? The most bare-bones and 'pure' way is by using software such as Adobe Photoshop, which can provide the most creative -- but intimidating -- solution. However, these days, there are plenty of free applications on the Internet, in smart phones, and elsewhere on the computer that can aid you.