The best way to do that is to download the image file and change them in fireworks then upload them again... This is the quickest way. Simple and it works!
All the images is one big file so you can create you own image and put whatever you want.
Don't forget this: you have to do the same thing for each color if you are going to give your members the option to change the template style color to blue, orange or pink... Should take you more than 10 minutes.
I hope this helps!
Tabs_big_icon.png
Tabs_small_icon.png
you have these files in each style folder...style1, style2 and style3.
Change the name of the files as you download them. something like this so you don't overwrite the file:
1Tabs_big_icon.png
1Tabs_small_icon.png
2Tabs_big_icon.png
2Tabs_small_icon.png
3Tabs_big_icon.png
3Tabs_small_icon.png
After you edit the image the way you want just upload them back to the style folder. Rename the image file on your server to something else just in case something goes wrong
mainfile_Tabs_big_icon.png
mainfile_Tabs_small_icon.png
now upload the files you edit. Once uploaded just remove the number from the name
3Tabs_big_icon.png
3Tabs_small_icon.png
becomes
Tabs_big_icon.png
Tabs_small_icon.png
make sure to place each set of image in the right Style Folder...
Thats it!
Again I hope this helps...