you type on a page you probably don't think about it but the white
'paper' is actually a transparent background.
Images, however, tend to be rectangles
filled with colours: the colours of your photograph or whatever
colour the software puts in for you in order to retain the horizontal
and vertical dimensions needed. That 'background' is usually white
and so doesn't show until you place it next to something else or
on a non-white background.
A gif image can have a colour made
to be transparent which can be very useful as these illustrations
When saving or exporting an image
in gif format there will be an option to select a transparent background.
On the Word picture toolbar you can specify a colour to appear transparent
too with similar effect.