Vector (EPS/SVG) or Bitmap (PNG)
Choosing which download is right for you
(Vector or PNG)
If you want to be able to alter, scale, recolor or reorder the design objects then the best choice is EPS/SVG format. You will need vector software such as InkScape which is freeware (SVG) or Illustrator (EPS) which is a commercial package. When using vectors you are able to scale the vector to the size you like without any loss of detail and export it later to PNG or JPG. More information on vectors at wikipedia (-Link-). A list of vector software (-Link-)
For people who don’t have any understanding of vectors and only want to use a design or background instantly the best choice is PNG. You can’t recolor or reorder the elements in the drawing but you can still add things to it (graphical objects or text) or when the PNG has a transparent background you can cut and paste the objects into your designs. There is enoumerous software which can be used for altering PNG images such as Gimp (Freeware), Photoshop, Coreldraw,… More information on bitmap at wikipedia (-Link-).