See if I understood what you want. Suppose your image coordinates go from 0 to 1.
If you do:
r = Sqrt[(x - .5)^2 + (y - .5)^2]
a = ArcTan[x - .5, y - .5]
rn = r^2.5/.5
And then remap your pixels according to:
x -> rn*Cos[a] + .5
y -> rn*Sin[a] + .5
You get:
You may adjust the parameters to get bigger or smaller bulges.
Edit
Let’s see if I understood your comment about warping. The following images are generated using
rn = r^k {k: 1 ... 2}: