z520a External Wallpaper

I have had my z520a for a few months now and love it! When I first had the phone, I could choose a picture to use as external wallpaper and it would fill in the whole screen. Suddenly one day, even using the same picture, it would not fill the whole screen! I can see whatever theme I have the phone set to in the background and the picture is really small! I don't think I changed any settings when it initally occured and have since looked all over trying to find out if I can edit the pictures somehow to make them larger. Anyone know how I can get the picture to fill the whole external screen?

go to settings
display
external wallpaper
pick picture theme or none
once you pick your picture
it will give you an option

standard
maximize

pick maximize

Durrrrr! That is how I was displaying my pictures at first but then I started doing it either through my files or photodj! Thank you velvetgirl so much for your reply, it was very helpful! Now I just need something for my memory! Yikes!