Default image used when variable is not found

For an HTML based Webpage ( No CGI access btw)
var mydate=new Date()
var day=mydate.getDay()
var month=mydate.getMonth()+1
if (month < 10)
var day=mydate.getDate()
var newimg=""+month+day+".gif"

document.write('<img src="`newimg`">`)
This script works fine, as long as I have an image with the date in the name of the image in the "img" folder,
for example a file called "today1225.gif" is an image for christmas
What I want is to change this so that if there is no image in the img folder to use on that day, it uses a "default.gif" instead.
I don't want to use something like an alt text, I'd rather have an alternative default.gif

I was hoping there is a way to do this without making a database in the script of every image in the folder.
A javascript equal too... document.write('<img src="./img/today"+month+day+".gif alt img="./img/default.gif">`)
but the command "alt img" isn't real.

Any Ideas?

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ZvonkoSystems architectCommented:
Do it like this:

document.write('<img src="'+newimg+'" onError="this.src=\'./img/default.gif\';">')


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