You can attach images from your PC when you are editing the car or mod details using the browse button. Maximum file size is 1024kb, and max resolution is 1024 pixels.
The option to link to a Remote Image using a URL for pics doesnt always seem to work, so just save them to your PC and attach them for now, i'll sort that out.
You can also go to Manage Vehicle Gallery and add / remove pics of the car there too.