Even wih the last nighty build, i've got the same problem, i think its server side but i dont know where to check...
My php.ini just to check :
max_execution_time = 300
max_input_time = 300
memory_limit = 256M
upload_max_filesize = 16M
Bug still appear on different picture size(2Mo/3Mo/5Mo)... And doesnt seem to be logic, sometimes on a new folder sometimes on existing..
But now tinyMCE works on Pages and News Admin Edit
Does anyone have an idea ?