The forum will scale the image to fit in the text area, so when I link to an image I put the url in an img tag, and then put those in a link, which points to the same url:
<a href=”smth.com/image.png”><img src=”smth.com/image.png” /></a>
I will usually then add target=”_blank” in the link, so that it opens in a separate tab:
<a href=”smth.com/image.png” target=”_blank”><img src=”smth.com/image.png” /></a>
In the panel for writing a post you can see blue tabs at top-right for “Visual” and “Text”, and you should probably use the “Text” mode, if you want more control. The wysiwyg “Visual” editor does various funky things with letting you set the size of an image, and so forth.