Use the brush to highlight areas of the image you want to preserve (people, animals, text, etc.). This prevents the resizing algorithm from inserting or removing pixels in these areas.
Remove Highlights
Options
Interpolation
Interpolation determines how new pixels are created when expanding the image. Generally Bicubic interpolation produces the best results however it is also very resource intensive.
No
Linear
Bicubic
Seam Reusability
Seam Reusability affects how likely the algorithm is to reuse previously found 'seams' of pixels in the image. 'High' setting will often result in visible stretch marks across the image. 'Medium' and 'Low' options try to mitigate that effect in their own different ways.