1. First check to make sure K2 component can write to image folders.
Joomla Admin > Components > K2 > Information. On right hand side you will see folders and make sure they show as writable.
If they show non writable use a ftp program and set those folders to 775 or 777 permissions.
2. Are there any error messages displayed on screen, if you can't see anything enable error reporting in joomla > global configuration > server > Error Reporting "Maximum".
3. Try lower resolution image, not in size but in lower resolution.
If image uploads but higher resolution images does not then increase following settings. You might need to speak to your host for these changes. You can check current values in Site > System Information > PHP Information.
php "max_execution_time" to around 300
php "IPCCommTimeout " to around 300
php "memory_limit" to 64
See you around...