You can't use a tif file in a web publication. Try jpg or png for best
quality.

Images that are optimized for the web are using resampled and optimized at
either 72 dpi or in the case of Publisher to 96 dpi. This is a far lower
resolution than you would use in a print publication.

Which version of Publisher are you using. There is a built-in image
compression feature in 2007, and you can add it to 2003. When you insert an
oversized image at a higher resolution into a web publication, and then use
the compress feature, Publisher will resample and resize the image to fit
the picture box at 96 dpi.

Reference: Compress graphics file sizes to create smaller Publisher Web
pages (2003):
http://office.microsoft.com/en-us/publisher/HA011266301033.aspx

Reference: Compress Pictures dialog box (2007):
http://office.microsoft.com/en-us/help/HA100363901033.aspx?pid=CL100605171033

Try using the compress graphics tool and do a page preview and see if the
quality is adequate for your needs. If not there is a tedious method that
might give you a slightly better image....but don't forget that your image
will look different on different monitors, so there is no such thing as
perfection. Most people find the compressed graphic ok.

DavidF