The easiest way to add a pic to a group is to create an
album, upload the picture(s) from your computer/phone to the album, then under the uploaded image click the "Upload to Group" button. Check the box of the group you want to add it to and then click "Add Pictures to Group".
If you want to add it to your post in a group, upload your picture to an album, under the uploaded image click the textbox for BB Code. Copy it and then paste it into your post.
If the image is already on a hosting site like Imgur or Photobucket, copy the image URL, then go to post a reply on this forum and click the image link button (3 buttons to the right of the little globe above where you type). Paste the image URL into the popup and click "OK".