Hi, when I add a image for an item, the image result is a little image size, 1x1px and 600 bytes.
imgur.com/oGL5sD0
I have Joomla 3.4.8 and K2 2.7.0.
3 days ago, I can add images, normally.... everything worked fine

Regards

Was there an update on your server setup?
Did you install a 3rd party extension?
Finally, is the error_log recording any errors?

No, to upload or browse server, the image size is 1px x 1px!
I change CHMOD to 777 in media/k2 folder, reinstall K2, but nothing....

error logs:
[31-Mar-2016 16:46:31 America/Chicago] PHP Warning: unlink(/home1/xxx/public_html/xxx/libraries/cegcore/cache/db_tables_info.cache.php): No such file or directory in /home1/xxx/public_html/xxx/libraries/cegcore/libs/cache_engines/file.php on line 83
[04-Apr-2016 14:26:41 America/Chicago] PHP Warning: Invalid argument supplied for foreach() in /home1/xxx/public_html/xxx/components/com_mailto/helpers/mailto.php on line 91

This create in k2/items/cache folder:
imgur.com/n47hysM

Regards

These errors are generated from 3rd party and core extensions.
Can you try disabling 3rd party extensions, starting with cegcore (?) to see if there is a conflict?

Furthermore if you have a dev version of your site or an older version available, can you check it as well?