I don’t seem to be able to post full size pics
Not sure which picture app you are using, if you are not using
https://imgur.com/ you may want to give it a try, it is very easy to upload pictures and post them on Forums in various sizes.
After uploading an image click on it and on the right you will see View and Edit Post.
Under the heading,
BBCode (message boards & forums) click copy. Before you copy, select the size you would like to post.
You will have to set up an account, it's easy and Free.
