I use postimg for adding images, but when I copy and paste, the image appears the same size, no matter if I try to resize it to message board size, for example below
Hidden Image/Members Only
You can change the size of an image by adding the width manually into the code & confirm via the preview button before posting.
For above see the addition made to your image code below in bold (ignore the extra spaces, I did this to stop the image showing)
[ url=http://postimg.org/image/3z0neqqoz/][img
width=300]http://s5.postimg.org/3z0neqqoz/image.jpg[ /img ][/url]
[/quote ]
Hopefully this is what you wanted?