Picasa is a great program for keeping track of your photos and performing enhancements. However, I don't believe it will do what you want.
I would recommend Adobe PhotoShop Elements or if you have PowerPoint, you can make up a slide, add pictures and text and whatever else and then save it as a jpg picture. The following are examples of before and after pictures of our city limit sign. I couldn't wait until spring for the sun to totally illuminate the sign without shadows so I made the sign look the way I wanted it to with PhotoShop.