Server requirements for uploading large images
Where do I have to place the php.ini so the upload script knows about it?


In the same folder that Craft's public index.php lives in should be the only place it's required.

You can put a phpinfo.php in that same folder with the contents &lt;?php phpinfo(); and load that page in the browser to see if your php.ini changes have taken affect.


  Also it doesn't depend on the file size but on the image resolution.


That's because there isn't a 1:1 correlation because an images size on disk and how much RAM is required to load that image into memory.

post_max_size, memory_limit and max_execution_time and upload_max_filesize are usually the common culprits when dealing with uploading large files in PHP.