this depends entirely on the kind of picture,
for photographs .jpeg is generally better than png because it results in smaller file sizes with negligible loss of quality. (this may depend on the program you use tho)
.png is lossless so there's is no loss of quality when you convert to it.
.gif generally offers slightly worse compression as .png with the same amount of information. BUT images will usually be reduced because .gif allows less depth of color. And thereby getting a smaller file size.
also there may be some differences in file size depending on which program you use to convert it.
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