PDF partial image capture and paste

Is there any rule to paste a part of a captured image from a pdf document? I found sometimes it works and sometimes not even if I made the same type of groups and rich text categories. Tried different image formats in vain. Am I the only one?

Using DTP 2.x beta…

Personally, I cheat by using an image capture app such as SnapNDrag. That lets me select the area of the image and define the captured image filetype, size and quality. I can either save the capture or copy it to the clipboard.

Open externally in Skim or Preview, and use the select tool. CMD-C copy the selection, go back to DT, use CMD-N to make a new image file with the clipboard. Maybe some day the select tool will work in DT, too.