Help:How to reduce colors for saving a JPEG as PNG
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Sometimes, you find a drawing or similar image useful for a Wikipedia article, that was saved as a JPEG but should have been saved as a PNG. JPEG is good for images where the color changes fluidly throughout the image, like in a photograph, whereas PNG files are good for images with relatively few colors, such as a drawing of a flag, a chart, or a map; note that sometimes SVG is better.
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Just automatically converting to PNG will almost never produce a good image. It is likely that the resulting file will be even larger than the original JPEG, and will maintain its poor quality.
However, with a good bitmap graphic editing program with which you are reasonably skilled, you can produce a good result. This tutorial uses the GIMP, but it can be used with most raster graphics editors.