In HTML you should set the ALT attribute on images, it's useful for when a reader is waiting for an image to be downloaded, they will see an explanation of what will be there. Likewise, some of your readers cannot see images, they may be blind, color-blind, low-sighted; the ALT attribute is of great help for them as it can be read out loud by their device.
In emails it's more or less a *MUST* to specify. A majority of your recipients will probably not show images in the emails by default. An ALT attribute will give you a chance to give them a message even if the image is not shown.
Remember to verify how your email looks without images; maybe you have specified too much text on an ALT attribute? Maybe it's not descriptive enough? Maybe the email looks a bit weird without images?
So, always specify the ALT attributes on all images, even if it is just a button.
In SendCastle's WYSIWYG editor you simply right-click on an image and go to "Image Properties" and set the text in the "Alternative Text" field.
In HTML, given the image (logotype):
You'd want to make it:
<img alt="SendCastle" src="http://www.sendcastle.com/image/SendCastle-138w.png">