Just open your image folder (e.g. "My pictures"), grab a supported graphics file and drag it right over to the front panel (or even diagram).
If the image is on a web page, right-click it and select "copy". Go to the front panel and paste it. Now resize and move as desired. Resizing typically distorts the image, but you can always go back by right-clicking and selecting "original size". 🙂
See also the links in:
For details, a list of supported formats, and OS specific issues.