If you look in your User CP you will find "pictures and albums" as an option on the left side. You can create an album there and upload photos. That is one option.
Another is to insert an image in a message. There is a button for it in the 'reply to thread' message window. It's the one that looks like a post card. However it requires a url for the image, which means you'd have to post it someplace else, such as in your forum photo album or on another website. When you post photos here, a url code will be generated that you can copy and paste. It is in a field below the photo. Such as this:
If you left click the image and select copy image location you will see it is a url for the forum album for Janfred's Falkland Islands album.