edit photo to fit dimension of 50 kbs

i am often told a file I want to use as my personal id in a web based account is too large, that it needs to be less than 50kbs. i take most of my pictures with my ipad and imported it to my I mac, the one i am presently trying to edit came in as 480 X 640 but- Evernote requires smaller file. I do not understand conversions. How do I shrink it to this size (50kb). I have already created a copy of the original.

By the way is there a safe way to remove all and I mean "all" of the duplicate files in my libray? it is like 3X what it should be. I use Picassa evey so often so I am thinking maybe this app is crerating another copy of the iphoto file to keep in its own libray or is it able to just create a map to the original? * I have the same issue with my itunes music, even after I say show duplicates which are most often not exact dups, I still have multiple files of the same song files...sigh

a lot to answer, bit any bit of light is much aoppreciated!

thank you,

jody