Hi,
The problem is not about the size of the image file in megabits but the size of the image in pixels.
When PHP loads the image into memory, the image file is decompressed as raw pixels, each one taking several bits. So the bigger the size of the image in pixels, the bigger it will take in the memory. The 14M jerome was talking about is the size of the decrompressed image data that is required for each image in the memory. As your memory limit is 64M, you can see how fast it can be a problem.
So changing the file from png to jpg won't change anything for your problem. You would have to reduce the size in pixels of the images. So if you have one or several images with a big size in pixels, that's the one(s) that you want to resize down.