Using Picasa's tab labeled "Commonly needed fixes," you can type custom text on any image in Picasa's library. After creating the text, you can move it around and customize features including its color, size and orientation. Picasa saves both your altered image and the original, so you can always remove all text later if necessary.
After double-clicking the picture you want to edit, click the "Text" button in the "Commonly needed fixes" tab, represented by an image of a wrench. Click anywhere on your image to type your text. When you're finished, click "Apply" in the left pane to save that text to your image.
To customize text, click on the text you want to change to open the customization options. Use the drop-down menus to change the text's font and size. Other options are also available including Styles such as bold and italic, opacity via the Transparency slider, and justification using the Alignment buttons. When you click on text, the rotation button displays beside it; use this to rotate the text to any angle. To remove all text, click any text box and then "Clear All" in the left pane.