Well, I had a 420x105 image placed nicely in my sig and everything, then I went to make a slight change in the text (nothing with the graphics), and it gave me that 400x150 error. So perhaps it was a bug or some such in the image allowing code, or something else entirely, I'm not really sure. But I guarantee you I had
this absolute monster of a signature in my sig up until recently. I can call witnesses if you like
Oh, and I'm only proposing an increase of around 25 pixels. Allowing for 150px height, that's 3,750 pixels. Now, a pixel in a JPEG image is around 1 byte in size, so that amounts to 3,750 bytes, or approx. 3.66KB. Now, assuming everyone decides to take total advantage of the change and supersize their sigs, that's an extra 73.24 kilobytes of data on the 20 posts-per-page setting.
Assuming dial-up transfer rates to be 56kbps, that amounts to around, oh, 12 seconds extra download time in the worst case scenario. Allowing for fossilized technology, the total increase in load and processing time wouldn't exceed a minute. And this is assuming you're running DOS or something, and everyone on ZU suddenly gets a maximum sized banner traded for their old one. See what I'm getting at?
And one final thought: would it be possible to place more lenient restrictions, but some limit on filesize instead? Because the graphic example I provided is actually smaller and takes less time to load than a 400x150 (total 60,000px) image, by virtue of being only 420x105(total 44,100px). Seems rather silly to me