The "sidelines" indicate that the picture isn't the same dimensions as your monitor (i.e. 1920x1080).
If you go to where your picture is stored on your HDD, you can see the dimensions by:
- Hovering the mouse pointer over it, a screentip should pop up with the dimensions.
- right click on the picture icon and select properties.
- If you have the "Details" pane on, you can left click on the picture and the dimensions should be listed.
Maybe these pictures will help.
This Windows 7 VM is set to 1440x900 screen resolution (Backgound colour set to Black).
Original Picture (600x600 - smaller than the screen in both dimensions)

Fill (picture dimensions are
increased to 1440x1440)

Fit (picture dimensions are
increased to 900x900)

Stretch (picture dimensions are
increased to 1440x900)

Tile (picture dimensions are
unchanged and it is repeated as many times as required to fill the screen)

Center (picture dimensions are
unchanged 600x600)

Since the original picture is smaller, than the screen resolution, "
Fill", "
Fit" and "
Stretch" increase the picture size.
If the picture was larger, than the screen resolution, "
Fill", "
Fit" and "
Stretch" would reduce the picture size.
If I edit the picture to match the screen dimensions, "
Fill", "
Fit" and "
Stretch" won't change the picture size.
Edited Picture (1440x900)

If the picture is too difficult to edit, you have no choice but to put up with it being distorted, or sidelines appearing.
Did you recently get a new monitor?
If so, your old monitor may have had a different screen resolution (e.g. 1600x1200).
If you swapped from a CRT monitor to a LCD monitor the behaviour may be different (i.e. a CRT can be set to any resolution, but a LCD can't be).