guys clean master doesn't clean the .thumbnail files it just cleans the junk files, residual files and obsolete apks.
For deleting the thumbnails you need to follow the following steps no need to download any app for it.
1) create a new folder on your desktop, inside that folder create a text document and rename it as .file
2) make as many text document files with extension as .file as there are .thumbnail files under your DCIM folder => .thumbnail files and rename the text document files with exactly the same name as .thumbnail files (or you can make some text document files and rename them again and again with the new .thumbnail files).
3) delete all the original .thumbnail files and replace it with the new text document files that you made with the extension .file
4) reboot your phone and you are done.