MyaMya said:
I was impressed by your kind explanation.
However, I don't use this site, so I don't know many things about it.

1. Where can I check other administrator than the checkmate?

2. What does it mean to flag?
I want to know how to do it.
Should I just add the "Artist taken down request" tag to the "Edit Tags" section of my work?

3. Is there any way to know the uploader who uploaded my picture?

I would really appreciate it if you could help me again.
I look forward to your warm help.
Thank you very much.
for 2. there is a "flag for deletion" link to the left side of each image under options. put a message on pixiv in the comments saying you "request takedown for this image on other sites". that way the mods know you are the original artist.