How do I upload a picture to this website?

To upload photos to your portfolio, login to your account and select "My Portfolio" from the top navigation menu and then select "Edit Your Photos" from the side menu (red buttons). Here you can browse the photos you wish to upload from your computer. Name your photo, select a category for your photo and choose if you want the photo to be a "Public Photo", "Private" or "Just For Fun". Select "Upload Photo".

There are many image types that images can be saved as. You say the type that casting360 require is jpeg. Your file is obviously a different type, or format. It may be gif or png or one of many other types. You can tell what type it is by the letters at the end of the file name - the (usually) three letters after the period. You can convert your image from whatever it is to jpeg (.jpg) at many online free services.

Here are a couple:

http://www.online-utility.org/image_converter.jsp

http://www.coolutils.com/online/image-converter/

Having converted it and then got the converted version on your computer, then you can upload it to casting360.