problems with preserving image quality

i hate to ask but i can't seem to fix this. up until now i've saved like 99% of my stuff as .png but today something weird happened. i did myself a new header which i liked and i saved it and uploaded it but what used to be a nice soft light brown header came out as a ugly darker brown header. i tried saving it as .jpg and .gif and .pic and various options but couldn't get it to look the same as when it was in my program (psp if it matters). none of the options made it look the same. they all came out darker brown when i uploaded it or saved it. this has really been frustrating me so if anyone can help i'd greatly appreciate it. thanks ahead!