Please read the appropriate documentation carefully. It will tell you what, if any, restrictions the author places on the use of the program. It may be just a standard disclaimer that you use the program at your own risk. It means you can use it on your own personal computer at home and you can tell your friends about it. You cannot imply that it is your product nor can you use it in your business.
Good graphics take a lot of expertise to produce.
It is amazing that so many authors give them away. If you use someone else s graphics do NOT allow others to think that it is your work. Give the Author credit and add a link back to the author's web page. Read the author s license agreement very carefully. The author would like to know a little about the kind of people who are using the program.