I'll try to clear this up for you...
As far as I've known, the sig pic dimension limit has always been 500x300 pixels. That is, it can't be longer than 500px and no taller than 300px. Maybe Rocky can clarify that, but I've never known anything different. If anything, the height was probably smaller at one time and increased to 300. You've been here basically as long as I have...Nothing has changed recently.
The rules say sigs must also be horizontal - longer in length than height. The way you have the pics in your sig right now isn't correct. Multiple pics like that should be side-by-side, not on top of one another.
I'm not sure why you think GIFs can be bigger...The dimension size is the same for all. The extra rule regarding GIF animations (whether it's an avatar, sig line, or posted as a pic in a thread) cannot be more than 500
kilobytes in file size.
I know there are probably a few sigs/avatars that are over the limits in the rules here, and they will be notified of it.
As far as imageshack is concerned, I'm not sure I understand the problem you're having. My sig is 500x150 in dimension, and I was able to upload it to imageshack without a problem.
Hope that helps. If there are any questions, from yourself or any member reading this, post it here, and we'll do our best to answer them.