i know i can use the alt tag for text in a image like so:
<img src="image.png" alt="alternate image text" />

but i am wondering if i can do a alternate image instead of text
so like if image.png doesnt load, then it would load image.jpg

"alt =" is NOT a tag, it is an ATTRIBUTE of the <img> tag. It is ONLY meant for 'alternate' TEXT. If you want to swap images, you are going to have to use another method, such as javascript.

Ladynred, I think he may be referring to a technique somewhat like the one we can see on http://www.deviantart.com
When they present a list of images, the placeholder have a background logo, which fade when the image screenshot load.

I have never looked at it, but I think they may have set a "background" rule on the class, thus applying this background.

Raptrex, is that what you are looking for ?

yes i guess thats a way to do what im looking for

well then, that's exactly what I've thought.
http://www.deviantart.com/
They have embedded the img tag inside a span, giving a background-image to the span:

<span style="background-image: url(http://sh.deviantart.com/shadow/alpha-000000/2.6667-0.35/150/134/logo.png);" class="shadow">
<a href="http://www.deviantart.com/deviation/47203842/?&amp;qh=special%3Apopular" title="Commission Blue Tiger Copic by *WildSpiritWolf">
<i></i>
<img src="http://tn1-3.deviantart.com/fs13/150/f/2007/022/d/1/Commission_Blue_Tiger_Copic_by_WildSpiritWolf.jpg" height="134" width="150">
</a>
</span>

That way, while the image screenshot loads, you see the span background.
I don't know how they match for the background size, but at least you have a starting point.

The background image in the Span is nothing more than a method of applying a drop shadow to an image. It is not an "alternate image"

Great idea if you want to completely kill your bandwidth or load time.