You can set an ALT attribute for an image by doing the following:

1) Create a new text item and enter the text you want to use for the ALT attribute.
2) Move and resize the text item so that it fits completely within the bounds of the image you want to attach it to.
3) Right click the text item and select "Order -> Send Backward". Repeat this until the text is behind the image.

Now preview your site. The text you entered should become the ALT attribute of the image. If you are using Internet Explorer you can verify this by holding the mouse cursor over the image for a couple of seconds - the ALT attribute should appear as a little "tool tip" over the image.