We prefer JPG's so you are good to go!
In reference to the questions about artifacts, I have not had anybody that's had any concerns with this.
I'm doing a little research and you're welcome to check out this link for more info: http://www.wikihow.com/Improve-Jpeg-Image-Quality
Since I have not had any feedback or concerns with jpg artifacts I believe this is ultimately subjective. Please let us know once you upload the file if you have any concerns regarding artifacts, etc. If you want us to take a closer look at any point to the project itself you can always email your project ID number to email@example.com.