What you could do Paul or srellim. Is to have a look at the URL of each ball image to modify it's size.
I'll take the orange one in srellim's blog post as an example.
Okay, lets take a look at the URL...
http://media5.wgt.com/assets/community/images/wgt/proshop/assets/balls/wgt/common
/orange/wgt_ball-orange_small.jpg?0.61.5198.32468_1018
If you can see to the left of the numbers at the end of the URL there is the word 'small'. I didn't know for sure but I took a guess that if I substituted the word 'small' with 'medium' or 'large' then I might be able to get a bigger image, luckily for me it worked.....
Here is the original 'small' image...
...and here are the images after I changed the 'small' to 'medium' and then 'large'...
It appears that the 'medium' one has rather a lot of white space either side of it so that might not be suitable without some cropping.
You can just resize whatever picture you want to use by selecting the image and clicking on the 'Edit image' icon above the posting box and entering your size choice into the dimensions boxes (best to use one dimension only to maintain the correct aspect ratio of the image) or if you are comfortable with delving into HTML you can edit it that way.