what is the original size of the image?
most likely, either the original image exceeds the max upload file size set in the php.ini file, or the memory needed to resize the image was more than the server makes available through the memory limit setting in the php.ini file.
try adding the following code at line 1 of the insert page:
if the server has not blocked the memory limit setting, this will increase the memory limit allowed.
Where some images are uploading, but others are not, it is a server setting that is preventing them.
see the following page for details on php.ini settings that effect file upload: