Artwork for custom button making should be "print quality" also known as "high resolution". Your artwork should be made to the correct size and should have a bleed (A larger area that folds around the back of the button) Download the template that matches the button size you have ordered. Templates and artwork instructions.
If you already have a file you can upload it below. jpg or .jpeg files are ideal and should be created at 300 dpi. If you take a 72 dpi file from the internet and change it to 300 dpi the results are not normally satisfactory. If you have sheets of buttons .pdf is a good format.