[I can't find an e-mail address in your profile or the blog that it mentions.]
I was going with 4x file size and assumed that bandwidth for images is linear in file size, aka no bulk discounts.
One of my points is that IOs have a cost too, a cost that is largely independent of file size.
To first approximation, disk IO capacity is proportional to the number of disks. (Yes, some disks, especially the flash ones, support a lot more IOs/sec than others.) If you're IOs bound, you either have spare disk space or can probably increase the amount of disk space at a sublinear price. (1.5TB drives are For some data transfers, AWS has a "per operation" charge in addition to a bandwidth charge. The latter is proportional to file size but the former is not.
My application has a lot of processing costs. Most of them are on metadata, not the images, so they don't grow with image size. I also do a lot of stuff with "thumbed" images - producing them is a function of file size but storing them and moving them around isn't.
My model is different from yours in at least two ways.
(1) I'm estimating some things.
(2) I'm using AWS and GAE prices. (I'm assuming the highest prices because if my app is getting enough use that I'm getting bulk discounts, I've got other problems.)
If I had a model that tracked my actual experience, I wouldn't listen to some bozo on a website....
FWIW, data in a db is significantly larger than the actual data.