I tend to change the file format of my photos frequently -- usually from JPEG to TIFF or JPEG to Adobe Photoshop's PSD format. Why would I do that? Usually, to preserve image quality.
Every time you edit and then resave an image in JPEG format, for example, it loses a little quality (depending upon how much compression is used in the resaving). A few file formats, though, like TIFF and PSD, don't inflict that kind of punishment. So if I need to edit a photo that was originally shot in JPEG format, I typically save it in the TIFF format before I start editing to lock in the image quality, as they say in the sandwich bag industry.
That's not the only reason I change file formats. These days, I take most of my photos in my camera's RAW format. But RAW files are huge and most people can't view them on their computer. So in order to share photos with friends, family, editors, or anyone else who needs a copy, I need to save the final, edited versions as JPEGs.
As you can see, there are some good reasons to occasionally change formats. Let's talk about what everyone should know about how, when, and when to change a photo's file format.
File format myth
Before we go too far, let me dispel a popular misconception. A lot of folks have heard about how saving JPEG photos can affect the image quality. Some people take that idea too far and worry that viewing a JPEG can be hazardous as well.
Just looking at a JPEG file doesn't affect its image quality. You can open the file in a photo editor, view it, and close it again without affecting quality -- just don't make any changes and click the Save button. If you don't plan to edit an image, you can safely leave it in JPEG format forever and the image quality will never degrade.