Why can I only put links to 3 pictures? I didn't upload they are links.
The number of images you can add to a post (whether uploaded or linked to) is dependent on your level of membership. Members are under no obligation to become supporting subscribers, but there are a few minor "perks" for an annual subscription, as seen here:
https://www.britishcarforum.com/community/pages/subscriptions/
For the "Free" membership, there is a 4-image limit, then 6 for Bronze, 8 for Silver, 10 for Gold and 15 for Platinum, etc.
You should be able to add 4 images, not just three, unless the site you are linking from has some limitation over which we have no control. The 4 images above are linked from a Flickr account by my "test user" who is only a regular registered member.
By the way, I recommend you use the "Insert Image" to add linked images rather that the "Insert Link". I've updated the "how to" video above (I just realized I had been linking to an old video). Hope this helps.