you could ask them to save it as a .png file with transparency OR last time i checked, GIMP allows the user to save the file as a .PSD which is photoshop compatible with editable layers.
If that fails next best ting is to go to refine edge with your "jagged" selection and choose soft/smooth edge move the slider til it looks a little smoother. that's what i do
If that fails next best ting is to go to refine edge with your "jagged" selection and choose soft/smooth edge move the slider til it looks a little smoother. that's what i do

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