Transparent area appears as a "chessboard"
Currently only GIF and PNG files may contain transparent areas. If a transparent image is placed on a web page, the background of the web site is visible through the image.
To remove solid background choose Image > Remove Background command:
Drag the slider to achieve desired effect.
If the image has no solid background you have to remove it manually.
Check "Transparent background" to work in transparent mode:
To erase an area use the "Eraser" tool.
To erase an area in "bucket fill" mode use the "Eraser" tool and hold CTRL key then click inside a solid area to make it transparent.
To save transparent PNG or GIF image choose "File" > "Save" command.