There are two types of balloons, latex and foil. Foil balloons are traditionally filled with helium, which is lighter than air, so they float. They usually stay inflated longer than latex balloons and can last for several days. You can print customised designs on foil balloons using screen printing, litho printing or an inkjet printer. Unlike latex balloons, foil balloons do not expand when inflated so it is easy to print a message or a design on it. (See references 1)
Use litho printing. This is cheaper than screen printing but the quality may suffer. Not only are the images less defined than screen printed designs, they can also smudge. With litho printing, paint is applied to a template before the template is pressed to the balloon surface. This type of printing can be done easily at home using a lithographic template, which is usually made of limestone or aluminium. (See references 1)
Apply the image using screen printing. Screen printing is more expensive than litho printing but produces better quality images with a more intense colour and sharper lines. You will need a special equipment, as screen printing transfers the image by placing a mesh screen and forcing the ink through onto the balloon's surface. However, like litho printing, once you have the right tools, this is something that can easily be done at home. (See references 1)
Print the image using an inkjet printer. Conde produces balloons that you can run through a top-feeding ink jet printer before cutting them out and filling them with air. The balloons are not filled with helium but instead are propped up by coloured plastic stands. This method of printing allows you to put any image on the balloon you want, including photos. (See references 2)
When screen printing more than one copy of the same image, make sure you don't allow water-soluble paint to dry on the mesh as it will become difficult to remove.
Screen and litho printing both involve paint, so always wear protective clothing to prevent your garments from becoming permanently stained.