The first thing to check is matching the aspect ratio (width/height) of the
print size you want in inches (6 x 4) with the aspect ratio of the picture
you have in pixels.
For 6 x 4, the aspect ratio is 6/4 = 1.5
To check the aspect ratio of the picture, place your mouse pointer over the
file name or thumbnail. A textbox will open giving you the dimensions in
pixels, something like 2550 x 1920. The aspect ratio is:
2550/1920 = 1.33
This will not work because the aspect ratios d not match.
The printer driver has two options:
Crop to fit: crop the picture (remove pixels) a little from the top and a
little from the bottom so the aspect ratio becomes 1.5 and it fills the 6 x
4 space but a little of the top and bottom of the picture will be missing.
Size to fit: reduces slightly the size of the picture to get an aspect ratio
size of the picture so it will fit on the paper size you selected, but now
you will have a white border.
I suspect this is what is happening, your print size on your paper 6.25 x 4
paper is 5.25 x 4:
5.25/4 = 1.32.
So I think you are trying to print an image taken from a digital camera
(most shoot at 4:3, aspect ratio is 1.33) and it does not fit your 1.5 paper
aspect ratio.
I don't know your printer or what software you print with, but you may be
able to change the option of "size to fit" to "crop to size". Obviously this
last option, automatic crop, may cut the feet or head of somebody.
The best solution if automatic crop is not satisfactory is to crop manually
with a photo editing software. With this you can crop at any aspect ratio
you want and place the crop over the area of the picture where you want it
and then print.