I just wanted to say, I managed to get it working by re-setting all the options to the defaults. Then the custom field for the images showed up, the image had uploaded to the media library, and it even published with the post (I had used this code
<?php if (function_exists('usp_post_attachments')) usp_post_attachments(); ?> from this support question: http://wordpress.org/support/topic/still-having-issues-getting-a-template-tag-or-custom-field-for-images?replies=10).
I realize now that I had my maximum image width and height set way too small (on the options panel, the default value was 1500 by 1500 and I set it to something like 638 by 300). Then when I went to test it, I selected any old photo on my computer and they must have always been bigger than that. My thinking was that setting the max width and height would resize the image to fit in the available space, which it obviously doesn't. So it seems if the image selected is too wide/high, it didn't get included at all and there was no custom field for it.
Perhaps for uninitiated people like me, it would be helpful if there was a little note next to the option in the setting panel explaining what will happen when you set the width/height too small (i.e. possibly making it not possible for users to submit images). Also, maybe an error message displaying to the user when they've selected an image that is too large? And it would be nice if images that are bigger could be auto-resized to fit in the available space, though there is probably a way to do that already that I haven't figured out yet.
Thanks anyway, I think the plugin is really great and I'm going to give it 5 stars.