HTTP error on Images wasn't happening before I installed wpmudev

Now when I upload images I'm getting an HTTP error that wasn't happening before - images are 2448 x 1836 at 150 dpi saved as PNG-8.
I added the following to my functions file:
add_filter( 'wp_image_editors', 'change_graphic_lib' );

function change_graphic_lib($array) {
return array( 'WP_Image_Editor_GD', 'WP_Image_Editor_Imagick' );
define('WP_MEMORY_LIMIT', '64MB');

which had helped me on another website, but I think it's an issue with one of the wpmudev plugins.
Thank you!