Possible to use String value as Variable?

I am relatively new to AS3 and have never attempted using Strings so I am not having any luck getting this to work and honestly don’t even know if it’s possible.

For example:

var varName:String;
varName = "vet"

//The next line should read
//var bitFile = new vet(100, 100);

var bitFile = new varName(100, 100); 
var lineart = new Bitmap(bitFile);

I currently have to re-write the entire bit of code for any different image that I am using. However, I would like to be able to just change a single variable (varName) when loading a different image.

Any help/insight would be greatly appreciated.