This is the same procedure you use to save any file, but the list of choices of file format might confuse you.  There are many kinds of file format, but in most cases you will want to save the final edit of your picture as a ".jpg " file.  JPEG is a compressed format optimized for Internet style graphics.  It will give you a nice picture with a reasonable file size. 

If you want to use a picture in Thred, you need to save it as a bitmap.  But do not use Irfanview's grayscale first.  Even though a bitmap may look like it is black and white, grayscale images are not 1 bit images, they are 256 shades of gray, which Thred can't use.  You must save them as 24 bit color (sometimes also called millions of colors) or black and white line art.  Irfanview offers grayscale instead of black and white. For most purposes this is better, but for embroidery,  you want 1 bit black and white line art, so you need to save the file in another program.   Graphics five covers using Windows Paint to save a black and white monochrome bitmap.

Graphics  One

Graphics  Two

Graphics  Three

Graphics  Four

Graphics  Five