Seems to be working now. I had to stick the brackets in the external file & also change its single quotes to double, and switch .dayKey to [dayKey] in the script:

    $.get('alldatestest.txt', function(data) {
        var picsObj = JSON.parse(data);
        alert(dayKey + ' ' + picsObj[dayKey]);
    });

- returns "0812 100315/5065359-R1-19-20_scaled.jpg"

This was also helpful: http://json.parser.online.fr/

Thanks Peter!

Edit: to clarify, here's the new "alldatestest.txt" -- I also had to wrap keys in quotes per above online parser:
 

{"0810":"images/CNV00016-interim.jpg",
"0812": "100315/5065359-R1-19-20_scaled.jpg",
"1004": "100315/5065359-R1-19-20_scaled.jpg",
"1008": "images/CNV00016-interim.jpg"}

And now you can browse back and forth through the Daily Bad Pictures*, thanks to stack overflow and their diabolical spell
 

Object.keys(obj).indexOf(key)

(you're not supposed to be able to do indexy stuffs on objects, but there you have it  (angel) )

http://www.designartcraft.com/photo/afbp.htm

*subject to buggery (NJ), breakage and general fail