Images can be made to appear to blend in with a page
if the limits of the image have the same colour as the page.
However if the browser dithers the page and the image
differently then the edges of the image may stand revealed
and if the page has a textured BACKGROUND
they certainly will.
A gif image can have one entry in its colour table designated
as transparent which will allow the background to show through.
The Unix program `giftrans' will set a nominated colour
to be transparent.
Left is one copy of the BACKGROUND image used on this page,
not necessarily aligned with the page background.
A background image should be much "weaker" than is often realised -
pale if the TEXT is black and
dark if the TEXT is white -
otherwise the text will be hard to read.
The image should tile, which is difficult to achieve if it
does not represent simple geometric shapes.
Most (all?) browsers do not print the background and so it is probable that no difference is observable between the faces when this page is printed.