The photo templates in iWeb allow you to easily arrange and display photos with captions on your website. If you set a very large font size or have several lines for your photo captions, the text may get cut off by the border area around the photo.
You can adjust the font size of text by choosing Font from the Format menu, and then choosing Show Fonts. You can change the number of columns and the height of captions using the Graphic Inspector.
For more information on adding photos and captions see "Adding photos and captions" in iWeb Help.