I got really annoyed with gettting rubbish theme on my phone so I found a Theme Creater. (Sorry this is only for phone who have new sony ericsson phone's.)
Step 1: Get the Program!
Click this link to get the program you need to create themes
Step 2: Make Your Theme
When you've installed it and opened it, select your phone. You can use any image but where it says a number (eg. 176 x 176) you need to make the image that size. I use paint to change the image size. What you do is open paint, go to the image tab then select attributes . Then make sure the units are in pixels and type in the numbers (eg. 176 and 26)