With Image Resizing in DFP, the biggest factor will be the memory_limit php setting.
DFP Uses the GD Library requires quite a bit if memory when resizing the image, especially if your image upload form contains multiple file fields and/or you are creating multiple different sizes for the images that are uploaded.
Try increasing the memory_limit setting to 256M, this should work for most images. the 16m limit of your second host will be problematic when using the image re size functions