Only if you've deleted your entire Thumbnails folder. You can run "texturecache.py Xd" to resolve any issues if you've removed files from the Thumbnails folder.
Texture Cache Maintenance Utility: Preload your texture cache for optimal UI performance. Remotely manage media libraries. Purge unused artwork to free up space. Find missing media. Configurable QA check to highlight metadata issues. Aid in diagnosis of library and cache related problems.