If thats the problem, i think i can fix it, here is too late, i will try to fix tomorrow and upload a hotfix, fingers crossed!
Viewing post in I found a bug in PixaTools v157
I tried turning on the fullscreen by pressing F in admin mode after setting it to Windows 7 compatibility. It doesn't work normally.

%%NOTE: I uploaded the Fullscreen ScreenShot
And I tried reducing the image size to 1920x1080 again. The problem you were talking about has been resolved, but there is a strange vertical line at the bottom.
This phenomenon is the same for 2x2,8x8.

Dithering is not generated properly in full screen mode.

The last image is a phenomenon caused when stored in Fullscreenmode.
If you fix the strange vertical stripes again, I think I can use it without any problems!
%%NOTE:All of this was tested with the v157 hotfix.
