Issue: Uploading Images Inconsistent

I have been having trouble understanding how images in the Image Namespace should be uploaded. It appears that if the attachment is explicitly placed in the article, the image information is not read. Therefore, I now let the images be automatically placed by the Attachment Manager.

However, I have one image that uploads as a thumbnail in the Image Namespace while all the others upload as full size. The limits we have in place for attachments are 800px x 1400px for jpeg. One of my 795 x 760 images gets displayed as a thumbnail. All the others are full size.

Fletcher Chronometer shows as a thumbnail while the nearly identical Dent Chronometer displays full size.

I also have not been able to get the thumb parameter to do anything useful when inserting images.

Is there some brief information anywhere on how this is all supposed to work, or do I need to search Mediawiki and try to interpret from there?