There are two basic ways to convert photos to a different format:
1- Use a software program which runs on your computer to convert the image.
2- Use a website service (many are available free) to convert the image.
If you are using a Windows computer, the Gimp for Windows is an open source photo/image editor which can convert images into different formats. To import images into Windows PhotoStory3, convert them into JPG format.
On a Macintosh computer, the built-in "Preview" application can be used to convert images into different formats: JPG, PNG, PDF, etc.