Marking Up Existing Documents, Posts, Pics, Etc. ~ Any Tips?

Something has been on my mind for awhile and I just thought, 'why don't you try posting to Craig's Classroom?'

I am wondering how to do things like circle, draw arrows and comment on existing media.

This would really be helpful on a number of fronts.  I am particularly curious about how to accomplish this in a PDF or Publisher file.  Altering a photo, blogpost, screenshot, etc. also would come in very handy.

I just have no idea where to go to access such tools and how to make this work in my day-to-day work (and blog) projects.

Any help would be appreciated.

Thanks all!