After importing a png or gif, (or anything by the look of things) it looks like Xara decides what image type is 'best for the web'. This has happened so many times in the past, where I end up with a black or white rectangular box behind the image. Most noticeable when used on top of a slideshow or when animated.
So, after I import the image, I right mouse on it and select 'Web Properties', then click the 'Image' tab, then choose Web image type PNG. This always cures the problem for me.
If you look at my latest website here, http://mojitosmexicangrill.com/ you can see a logo on top of a Xara slideshow, which the logo itself is animated and leaves no artifacts around it. I have used this logo all over this website like this.