I want to check whether the images is downloaded completely. Is there any library to use?
The images I want to verify including various formats such jpeg, png, bmp etc.
You can guess by attempting to load the image into memory (using PIL or somesuch), but it's possible that some images could be loaded ok without being complete - for example an animated GIF might load fine if you have the header and the first frame of the animation, and you won't notice that later frames of the animation were missing.
A more reliable approach would probably be to use some out-of-band communication, like rather than watching a folder and processing new files as soon as they exist, find some way of hooking into the downloader process and getting it to give you a signal when it decides it is ready.
How to verify the integrity of a image file in c++ or python?