Not sure what type of photo you are referring to (bmp, jpeg, gif etc.) but
if you RIGHT click on the particular photo file you are wanting to edit, when
the drop down menu appears click on "PREVIEW" When that screen comes up click
on the little green Icon (First Icon to the left of the blue "?" at the
bottom of the screen) and see what program it takes you to to edit your
pictures. What ever that program you have on your computer "MIGHT" have an
option to change the brightness. Most users have some type of program on
their computer that they use for editing photos, not including MS's "Paint"
program.
Redwagon...