The delta between the image size in your photo editor and the size of the Roblox window is what’s causing this. I recommend working in a 1920×1080 canvas, and in Roblox, click on device preview and select that same resolution. Now all you need to do is make sure image size is always in sync, apply the scaling, and you’re good to go
Also, it is important to make sure the image is cropped to the boundaries when exporting!!
If your using the scale of the size Udim2, you need to change how it scales, via the SizeConstraint property, because your likely not changing the resolution the same amount in all directions. It should be set to XX or YY OR use the offset of the size Udim2, so its based on pixels, and not screen percentage.