What tool (Class) would I use to select images and take a snapshot of image in PDF for copy/paste into another document?
To get a snapshot of a PDF page you could use PDFDraw.GetBitmap() or PDFRasterizer (as shown in PDFDraw sample project - http://www.pdftron.com/pdfnet/samplecode.html#PDFDraw)
Regarding the GUI part you would need to implement a custom tool that draws a rectangle (or captures points). Please take a look at provided ‘tools’ sample code for concrete of how to do this part.
Once you have 2 coordinates in clinet/screen coord space you can obtain the underlying page with PDFViewCtrl.GetPageNumberFromScreenPt(x, y). You can transform a point in PDF page coord space using PDFViewCtrl. ConvPagePtToScreenPt(x, y, page_num).