When you say open in documents, what exactly do you mean. By that I mean which program do you usually to view them, paint, word, windows photo viewer.
If you right click on a pic,you should see a list of options, including open with. Click on that and you should see more options, at the bottom is an otion to change default. Click on that, and you will have more options, select the one you want, or browse to the prog you want to use and select it
Picassa may have asked you if you wanted it to be the default prog at some stage, and you must have clicked yes. Its a good idea to carefully read the options presented when you install new or updated software, otherwise unwanted changes can occur.