I have had this same problem. First time using Silverstripe, I have to say it is awesome but I am having trouble with this issue.
Here's what has been happening:
1. I installed Silverstripe on my server, worked great but realised I had published it into a folder called Silverstripe so deleted everything to start again.
2. Second attempt at installing it I noticed that the Silverstripe logo didn't fully load in and looked to be corrupted, then when I accessed my pages in the control panel the tiny mce toolbar is corrupt. I have downloaded the latest version and even tried to upload these two images but to no avail.
Now here's is where it gets a bit strange. I have tried some of the suggestions on the forums ie [/dev/build/?flush=all or admin/?flush=all], still no joy. Out of interest I wanted to see what would happen if I completely fresh installed on another domain. The exact same issue has arisen, the images even produce the same corrupt look.
This to me sounds like a browser cache issue, the thing is I get the same thing every time I clear my cache, so I tried to log on in my work place just to make sure I haven't got some crazy caching issue and get the exact same thing.
So could it be my server? Even when I have installed a copy fresh on a completely different site?
Any help much appreciated