Imprime en el else, la variable $_FILES['imagen']['error'].
Posibles valores que devuelve esta variable:
Cita: 0 Indicates that there was no errors and file has been uploaded successfully
1 Indicates that the file exceeds the maximum file size defined in php.ini.
If you would like to change the maximum file size, you need to open your php.ini file, identify the line which reads: upload_max_filesize = 2M and change the value from 2M (2MB) to whatever you need
2 Indicates that the maximum file size defined manually, within an on page script has been exceeded
3 Indicates that file has only been uploaded partially
4 Indicates that the file hasn´t been specified (empty file field)
5 Not defined yet
6 Indicates that there´s no temporary folder
7 Indicates that the file cannot be written to the disk