When you use an image as a mask for the custom Word Search, it needs to be a solid dark color in the shape you want on the board.
Something like Black, Dark Red, Dark Blue, and have a white background is most ideal.
These examples below are the types of images you should use, they have solid colors that are clear and defined. (Keep in mind petty much with no matter what you use, you may need to click on a square to add to the shape, or subtract a square from a shape).
These shapes below are the types of shapes that can not be used and just will not come out well on the board.
So please use shapes with solid colors and a white background is the most ideal.
Shapes with just outlines will not work either, like these below ones, because they have no filled in solid area.