Use the Polygon Lasso Tool to roughly outline the desired area of the image.

Enter Quickmask mode Tool box (bottom left) to clean up selection area.

Draw mask: black creates mask, white (or eraser) removes mask. One strategy is to draw around inside edge  of object with paint brush then fill inside with paint bucket 

Return to standard mode 

Make figure current selection rather than background  Select Menu > Inverse 

Copy figure to the clipboard 
Edit Menu > Copy

Select desired destination image in which to paste figure 
Edit Menu > Paste 
or for figure on transparent background 

Create new file with unique color as background and export as gif89 (see below).