Rosey is back! Yey! :x...
You shouldn't have to worry about overwriting image files, unless you've made customizations... If you make customizations I recommend creating your own directory, and using that with your theme.
So long as you dont overwrite these folders, you should be fine:
cache_builders/custom
includes
styles
Thus, your stylesheets won't be over written; the upgrade system however will update the styles installed.
I would do the upgrade, THEN mod; you don't want things to go wonkers due to a miss-applied mod.
