Media size settings don't seem to change anything

I've tried changing the settings, but the largest picture I can upload is 800x600, and the largest MP4 video is 640x480. If the image sizes are bigger than this, then although it appears to upload, the post does not include the image or video. I've tried disabling other plugins, but it makes no difference. The support settings are: PHP 5.4.19 MYSQL 5.1.67 WordPress 3.6 BuddyPress 1.8.1 rtMedia 3.0.13 OS Linux Imagick Not Installed GD bundled (2.1.0 compatible) [php.ini] post_max_size 8M [php.ini] upload_max_filesize 40M [php.ini] memory_limit 90M

Sorry, I'm new to this and may be overlooking something really obvious, but I'm stuck.

The media upload does not depend on the Image/Video dimensions. They depend on the physical size of the media. From the looks of your debug info. No media larger than 8MB would be allowed on your site. The media size settings are just for the front end display.