How safe/reliable is flickr?

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Have just looked at it, after being pointed to it by a number of posters on dpreview.

I noticed that you can copy anyones picture - is there any way to stop that on this site?

do you find it beneficial to have your photos opened tothe public or do most of you keep them private?

When you upload, does the file size automatically reduce or do I need to do this before uploading?

thanks for any comments