The memory required to load an image using imagecreatefromjpeg() is a function 
of the image's dimensions and the images's bit depth, multipled by an overhead.
It can calculated from this formula: 
Num bytes = Width * Height * Bytes per pixel * Overhead fudge factor
Where Bytes per pixel = Bit depth/8, or Bits per channel * Num channels / 8.
This script calculates the Overhead fudge factor by loading images of 
various sizes.

Limit: 50M
Usage before: 385.74 KB

Loading image image1.jpg, size 1648 * 1236, bpp 8... done
Memory usage: 386.33 KB
Difference: 600 bytes
Difference / (Width * Height * Bytes per pixel): 9.8187073868099E-5
Destroyed. Memory usage: 386.7 KB

Loading image image2.jpg, size 824 * 618, bpp 8... done
Memory usage: 386.7 KB
Difference: 0 bytes
Difference / (Width * Height * Bytes per pixel): 0
Destroyed. Memory usage: 386.7 KB

Loading image image3.jpg, size 3296 * 2472, bpp 8... done
Memory usage: 386.7 KB
Difference: 0 bytes
Difference / (Width * Height * Bytes per pixel): 0
Destroyed. Memory usage: 386.7 KB

Mean fudge factor: 3.27290246227E-5