Uploaded Images Not Displayed

I've started posting to a new site that's still under construction. However, my uploaded graphics (.png) will not display in the post nor in media library. All I get is a "?" displayed where the image or thumbnail should be. Images uploaded to the same site months ago are also not being displayed in the library although they're still listed.

I searched the WPMUDev forums and didn't find anything about this, but an internet search suggested it's a permissions issue caused by the WP uploader. But I also tried uploading the files manually via FTP and still got the same result.

Is this a permissions problem or could it be a .htaccess problem. I've been trouble-shooting the later. See my post # 387082.

Any ideas?