In case anyone has problems getting image uploads to work (white screen), one thing that worked for me was enabling the File Browser Plugin that is built into Elgg. It turns out there is a class and function in that plugin that need to be called.

When the plugin is disabled, and you get a white screen, you are likely getting an error 500 because the plugin is unable to find the class "FilePluginFile", or the function "get_general_file_type".

If this doesn't work, open the .HTACCESS file, and change
php_value display_errors 0
to
php_value display_errors 1

...and you should be able to get some good details as to what is failing.

Hopefully this helps someone!