Detect A Not Defined Script Variable?

I've a picture gallery with the images stored as variables in a javascript file like this:

var a100image10 = new mypicture("album1/a100image10.jpg");
var a100image11 = new mypicture("album1/a100image11.jpg");

I've a "next" button to show those images (one by one) in a div with this code:

document.getElementById('image').innerHTML = "<img src=images/gallery/"+eval(album+"image"+next+".getPath()")+"></img>";

where the eval function returns the path of the pictures.This works fine! but in IE I get an error saying a100image12 not defined (and image13, image14, etc), because that image is not in the javascript file.So, how can I test in advance if a variable exists in the file?? Note that I take the name of the next picture based on the current one.I mean,I take the current picture from the div, lets say a100image10, I add one, and I try to look in the file for the variable a100image11.