You can only send jpeg files to our photo partner for printing. A jpeg file is just a specific type of image file.
The best way to check that your image file is a jpeg file is to look at its file properties.
On a PC
You can usually access the file properties by right clicking on a file in Windows Explorer and selecting the Properties option from the menu.
In the example, above, you will see that the file is called screen1.JPG and it has a file extension of .JPG and listed as Type of file: JPEG image.