Then you can use gimp, or Medibang/Firealpaca like the ones I mentioned before. These require a download.
Or, you can use sites like ImageResizer
that let you pick which colors you want transparent. (I recommend making the background a color not used in the drawing itself. i.e. for a white pony with a blue mane, use a lime green bg.) But when using ImageResizer, make sure the slider is on at least 2. (It's a little buggy)