class.upload.php is a powerful and mature PHP class to manage uploaded files, and manipulate images in many ways. The script is available under a GPL license.
As I said, you are very likely running out of memory.
Your destination image is 1600x1600. This requires at least 1600*1600*8 which is about 20MB. Assuming your source image is 2000x2000, you will have to add at least 30MB. It is already 50MB just for the images to be in memory. To this, you add the memory required by your project, etc...
An easy test to see whether it is memory or not: change your destination sizes to 160x160, and try to upload an approx 200x200 pixels image. If this one uploads and resizes properly, but the larger images don't, then it is a memory issue.Reply
Your destination image is 1600x1600. This requires at least 1600*1600*8 which is about 20MB. Assuming your source image is 2000x2000, you will have to add at least 30MB. It is already 50MB just for the images to be in memory. To this, you add the memory required by your project, etc...
An easy test to see whether it is memory or not: change your destination sizes to 160x160, and try to upload an approx 200x200 pixels image. If this one uploads and resizes properly, but the larger images don't, then it is a memory issue.