Choose the size you want. For the forum, small,medium or large is best depending on what you are posting. Original pix are sometimes giant and don't fit the forum screen.
small square looks good in your signature.
Use the BBcode "copy"---Click on COPY
Just Paste (CTRL+V on a PC, COMMAND+V on a MAC) it into your post.
No need to use the Insert URL function here. Just paste.