Dear Xara...

I admit I have never used Image Slicing.

Today, I found you cannot do this if the objects overlap.

If you set an object's Web Properties > Image > Web image type to JPEG, this is ignored.
It is all or nothing in the slicing Dialog where, at least, you can pick JPEG, PNG, BMP & GIF.
Sadly, no SVG.

Bug - if you chose an export path other than the initial offered one, a preview cannot be found.
Failed to open web browser onto web address 'untitled.htm'. Please make sure the web address is correct and complete.
Acorn