There can be several reasons for a file being "locked." John mentions the major "unlocking," Attributes. However, there are other factors that can play into things. One is another app. having the file open. One is when the file is on a networked drive, and adequate privileges exist. Another is Windows Indexing, which will lock files, while it tries to gather data on it. Unfortunately with image and AV files, it really cannot get the info that it wants, but that does not stop if from trying and trying, keeping the file locked, while it works on it. The larger the file, the longer it stays locked.
There is at least one file "unlocking" utility, Unlocker, that can address many of the causes.
Please tell us more about the file(s), their location, and whether you have it on a networked drive, or have Windows Indexing turned on.
Good luck,
Hunt