Set options for pasting pictures
When you paste some types of pictures other than a GIF, JPEG, BMP, DIG, PNG, or WMF into a web page and then save the page, Microsoft Expression Web automatically attempts to convert the graphic to a GIF or JPEG. By default, the picture is converted to a GIF if it has 256 bits of color or less, or to a JPEG if it has more than 256 bits of color. You can change these settings and other options in the Picture tab of the Page Editor Options dialog box.
On the Tools menu, click Page Editor Options.
In the Page Editor Options dialog box, click the Picture tab.
To define the way a file type is converted by default, click File Type Settings. In the Picture File Type dialog box, select the options you want, and click OK.
In the Page Editor Options dialog box, on the Picture tab, under Default File Type Conversion and Paste Settings, specify the default settings that you want, and then click OK.