* DEV: Fix png optimization test flakyness
Update fixture with oxipng 7
This test broke when the pngoptimizer got better so the pre-optimized png in the fixtures was compressed further on upload creation, breaking the expected size.
This feature ensures optimized images run via pngquant, this results extreme amounts of savings for resized images. Effectively the only impact is that the color palette on small resized images is reduced to 256.
To ensure safety we only apply this optimisation to images smaller than 500k.
This commit also makes a bunch of image specs less fragile.