You can read my post:
http://yousfi.over-blog.com/2016/05/a-cool-desktop-zones-captures.html
can capture any adjustable zone of desktop with gdiplusX library and save in any format you prefer.
it uses the standard format follow and without any fired dialog box.
lcdest is the destination bitmap file choosed:

 loCapture.Save(m.lcdest,.imaging.imageformat.PNG)  &&capture as PNG bitmap
or
 loCapture.Save(m.lcdest,.imaging.imageformat.jpeg)  &&capture as JPG bitmap

or
 loCapture.Save(m.lcdest,.imaging.imageformat.BMP)  &&capture asBMP Bitmap

or
 loCapture.Save(m.lcdest,.imaging.imageformat.GIF)  &&capture as GIF bitmap

can find in my blog many other relative posts talking of your question (activate search)