I think we can say Photoshop officially sucks at file size estimates then I remember someone else here had a similar problem with their sig not too long ago. The size it says out of photoshop and in Explorer is the correct size.

Avatar limit here is 5120 bytes, and 60x60 pixel image, not 5125 bytes You probably need to reduce the quality of the jpeg (but you can keep the image at 60x60), you've set it too high, and it really is pointless being set that high when the image is so small. I find a jpeg of pixel noise and random garbage at 60x60 pixels when saved at 90% of the maximum gives under 5k, so you've clearly set something wrong if you can't get it that small for something that isn't randomised. Alternately, try saving in gif format, the board accepts that as well.

If all fails, attach it and someone here can fix it for you