Hello @Twinsmama :)
You can take pictures of your build using the camera in the game. Press C to take the picture. To post the pictures, upload your photos to an image hosting site such as photobucket, Imgur or Flickr. Your game photos will be in your screenshots folder in your My Documents, sims 4 folder, and so upload these pictures to the photo site. Then copy the image code of the photo. Then, press the little file symbol that is above your post. Paste the image code into it. Press enter, and the link will appear in your post. Then press "post comment" and the picture will appear in your thread.
I hope that helps :)