the problem is caused by the memory needed to by the server to resize the images. IT has little to do with the file size. The error is a memory limit error. the server is needing to use more memory than is available to resize the images.
for resizing, the upload script uses the GD library which can be memory intensive, especially as you say the images have a high resolution.
it s a pretty clear error message:
Fatal error: Out of memory
the server has run out of available memory trying to resize the images.
The line of code i provided should increase the amount of memory the server will allow php to use.
If it is not working, there are 2 possible reasons:
1) The server does not have that much physical memory installed
2) The host has turned off the ability to set the memory limit through the ini_set command.