@eepjr24, @Ofnuts: Thank you all so much! 
My sister just read this and will try your quoted instructions, @eepjr24, on Monday. 
edit: My sister started with a bmp and then changed that bmp into an xcf. @Ofnuts, you didn’t mention bmp, but nevertheless, the transparency must have been introduced by the xcf. Right?
My sister just read this and will try your quoted instructions, @eepjr24, on Monday. edit: My sister started with a bmp and then changed that bmp into an xcf. @Ofnuts, you didn’t mention bmp, but nevertheless, the transparency must have been introduced by the xcf. Right?