Is it configuration of my settings, or a permission level set by the administrators?
SDMB doesn’t host images (except for profile pictures). You can link to images hosted elsewhere online. If you put the URL on a line alone, with no other text, it will show up as an inline image.
Sometimes putting in the URL doesn’t work, for a variety of reasons that prevent Discourse from either properly parsing the syntax of the URL (in which case converting it to a “tinyURL” might work) or parsing the actual contents of the link. In all cases, you can just upload the image to an image hosting site and link that.
This is what happens when I do that. How can I get the entire image to show (without clicking on it)?
Go to the imgur page for the picture, right-click on it, choose “Open picture in new tab” or something similar, depending on your browser, go the new opened tab with only the picture, copy the url of this page and paste it here in the edit field on a line of its own: