lol, it didn't show up but it posted, which means that at least the limit is correct.
Assuming each pixel is converted as RGBA, and the text limit is 20480, then the maximum amount of pixels I can have in a ctrl+v image is 5120. Which is approximately the amount of pixels in an image 71 x 71 pixels image (minus anything I type in the post).
In the post above I pasted this: 
(48x44). So it worked, I could post, but funny enough the image didn't come through