.Trace the edge of the image you want to extract.
Make sure the image’s edge is within the thick border that appears as you trace.
Connect the end of the border back to its beginning, or stop dragging to connect the border in a straight line back to the beginning.
Do one of the following:
To copy the image so that you can paste it in a document, choose Edit > Copy.
To remove everything outside of your selection, click Crop or choose Edit > Invert Selection, then press Delete.
Extract a PDF page as an image
You can select part of an image opened in Preview, then copy and paste it elsewhere or crop the content surrounding it. If the image is in a PDF, you’ll need to extract the PDF page as an image before you can use the image markup tools.
Open the PDF file, choose View > Thumbnails, then select the page in the thumbnail sidebar.
Click File > Export, type a name, Take a picture of your screen