I tried another user and same deal.
I changed my lock screen often while finding one I liked.
I did a sfc /scannow as admin in powershell and saw a lot about img100.png in there:
2012-12-13 06:27:09, Info                  CSI    000006c2 [SR] Beginning Verify and Repair transaction
2012-12-13 06:27:09, Info                  CSI    000006c3 [SR] Verify complete
2012-12-13 06:27:09, Info                  CSI    000006c4 [SR] Repairing 1 components
2012-12-13 06:27:09, Info                  CSI    000006c5 [SR] Beginning Verify and Repair transaction
2012-12-13 06:27:09, Info                  CSI    000006c6 Hashes for file member \SystemRoot\WinSxS\amd64_microsoft-windows-themeui-client_31bf3856ad364e35_6.2.9200.16384_none_c60cdb25defbc594\img100.png do not match actual file [l:20{10}]"img100.png" :
  Found: {l:32 b:ZvCpsup3yQreFvfjLMMfy9mWJK3h9GxpS5Hp+M+U6JQ=} Expected: {l:32 b:uY9CPvRWct/Qjjbx7SJ5uKSnx5ejSjNJUHv25oqV64w=}
2012-12-13 06:27:09, Info                  CSI    000006c7 [SR] Cannot repair member file [l:20{10}]"img100.png" of Microsoft-Windows-ThemeUI-Client, Version = 6.2.9200.16384, pA = PROCESSOR_ARCHITECTURE_AMD64 (9), Culture neutral, VersionScope = 1 nonSxS, PublicKeyToken = {l:8 b:31bf3856ad364e35}, Type neutral, TypeName neutral, PublicKey neutral in the store, hash mismatch
2012-12-13 06:27:09, Info                  CSI    000006c8 Hashes for file member \SystemRoot\WinSxS\amd64_microsoft-windows-themeui-client_31bf3856ad364e35_6.2.9200.16384_none_c60cdb25defbc594\img100.png do not match actual file [l:20{10}]"img100.png" :
  Found: {l:32 b:ZvCpsup3yQreFvfjLMMfy9mWJK3h9GxpS5Hp+M+U6JQ=} Expected: {l:32 b:uY9CPvRWct/Qjjbx7SJ5uKSnx5ejSjNJUHv25oqV64w=}
2012-12-13 06:27:09, Info                  CSI    000006c9 [SR] Cannot repair member file [l:20{10}]"img100.png" of Microsoft-Windows-ThemeUI-Client, Version = 6.2.9200.16384, pA = PROCESSOR_ARCHITECTURE_AMD64 (9), Culture neutral, VersionScope = 1 nonSxS, PublicKeyToken = {l:8 b:31bf3856ad364e35}, Type neutral, TypeName neutral, PublicKey neutral in the store, hash mismatch
2012-12-13 06:27:09, Info                  CSI    000006ca [SR] This component was referenced by [l:268{134}]"Microsoft-Windows-Client-Features-Package-shell~31bf3856ad364e35~amd64~~6.2.9200.16384.Microsoft-Windows-Client-Features-Package-shell"
2012-12-13 06:27:09, Info                  CSI    000006cb Hashes for file member \??\C:\Windows\Web\Screen\img100.png do not match actual file [l:20{10}]"img100.png" :
  Found: {l:32 b:ZvCpsup3yQreFvfjLMMfy9mWJK3h9GxpS5Hp+M+U6JQ=} Expected: {l:32 b:uY9CPvRWct/Qjjbx7SJ5uKSnx5ejSjNJUHv25oqV64w=}
2012-12-13 06:27:09, Info                  CSI    000006cc Hashes for file member \SystemRoot\WinSxS\amd64_microsoft-windows-themeui-client_31bf3856ad364e35_6.2.9200.16384_none_c60cdb25defbc594\img100.png do not match actual file [l:20{10}]"img100.png" :
  Found: {l:32 b:ZvCpsup3yQreFvfjLMMfy9mWJK3h9GxpS5Hp+M+U6JQ=} Expected: {l:32 b:uY9CPvRWct/Qjjbx7SJ5uKSnx5ejSjNJUHv25oqV64w=}
2012-12-13 06:27:09, Info                  CSI    000006cd [SR] Could not reproject corrupted file [ml:520{260},l:50{25}]"\??\C:\Windows\Web\Screen"\[l:20{10}]"img100.png"; source file in store is also corrupted
2012-12-13 06:27:09, Info                  CSI    000006ce [SR] Repair complete
2012-12-13 06:27:09, Info                  CSI    000006cf [SR] Committing transaction
2012-12-13 06:27:09, Info                  CSI    000006d0 Creating NT transaction (seq 2), objectname [6]"(null)"
2012-12-13 06:27:09, Info                  CSI    000006d1 Created NT transaction (seq 2) result 0x00000000, handle @0xdc4
2012-12-13 06:27:09, Info                  CSI    000006d2@2012/12/13:11:27:09.875 Beginning NT transaction commit...
2012-12-13 06:27:09, Info                  CSI    000006d3@2012/12/13:11:27:09.875 CSI perf trace:
CSIPERF:TXCOMMIT;5920
2012-12-13 06:27:09, Info                  CSI    000006d4 [SR] Verify and Repair Transaction completed. All files and registry keys listed in this transaction  have been successfully repaired