As a follow-up to Things to Remember: Why cells with an inserted image are taller than the image in GWT, the answer is:

VerticalPanel panel;
...
Image image = new Image("images/mydot.png");
DOM.setStyleAttribute(image.getElement(), "display", "block");
panel.add(image);

For whatever reason, the image object has ‘inline’ formatting by default, and when GWT assembles the table cell, the cell’s height is being derived from the font height rather than from the image height. Setting the image to block seems to resolve this issue.