Failed to get the image of sharepoint. Procedure

Hi, this is my first time Microsoft development, I am having a lot of problems, I am currently working on a project about powerpoint plugin, I want to get all the images in a folder in sharepoint, but I always can't get it, always report permission issues, 403 or Error: The partner returned an incorrect login or password error. Or internal exception :IOException: Failure to read data from a transport connection: The remote host forced the closure of an existing connection

But I can use this account and password to access folders and images in sharepoint. I don't know how to solve this problem.
I was hoping to get your help. Thank you very much!