It can be several things. First, the height and width of a picture don't mean much in download speed. The size of the picture in bytes or Kbytes is one of the things that will determine the time it takes to load. If you point at a picture with your mouse and hit the "right" mouse button and select "Properties" you can see the size in bytes. Obviously the larger the number, the longer it will take to load. You might even see the same image two different places that are different size files. This is because of the way it is stored. The signature clipart guideline are there just to try and control the size of all the images that get loaded to your computer every time you open a thread.
Another thing will be where the image is stored. The images are pulled from other computers as they are displayed on the page. Just like the connection speed at your computer will effect the download time, the connection speed of the computers where the images are stored will also effect the time it takes to get it from that computer to your computer.
One other thing is just the traffic on the Internet. You might have noticed that a page that loaded quickly yesterday is slow today. That could just be due to traffic and that can change even while you are loading a page or a picture.
Sorry so long. I hope this answers your question.