Liam, when you are looking for graphics on a Moodle page or any webpage and where these graphics are saved on the server you may use the info you get with your browser:
_ Firefox
Please open the dialog "Page Info" with the menu "Tools/Page Info" and click on the tab "Media". There you see a list of all loaded graphics with the "Media Preview" below for the selected one.
You may scroll through the list and check with the "Media Preview" which graphic you see on the Moodle page. In the list you see the file name and location on your server. For the theme "standardwhite" the gradient path used in the sideblocks looks like "http://yourserverandmoodleurl/theme/standardwhite/gradient.jpg".
When you change this graphic file you will get the gradient you wish.
_ MSIE
There should be a similar dialog in MSIE.
I hope this helps
Urs