How do I make a spray into a Bitmap image?

I have just found out that gmod only accepts Bitmap images as sprays. How do I make an image into a bitmap image?

Photoshop, or Windows Paint?

CouldI have a tut please?

Just openupin Paint,PS or GIMP and save as .Bit i think

Ok thanks I will try.

the extention is bmp not .bit

Do the File;
save as