There is an image in a paper that I want to use in my master’s thesis. I have the PDF file of this paper and I cant save the image from the file. So I have to take a screen shot. How should I cite the image?
You don't cite the screenshot, you cite the paper. If your readers want to look up the source, they don't care how you technically copied the image into your word processor. They want to know where they can find the paper you copied the image from.
It is very much possible and easy to extract the image from the PDF document. Please use: http://www.pdfaid.com/ExtractImages.aspx or: https://www.pdf-online.com/osa/extract.aspx or any other tool you can find online by searching for: 'extract images from pdf'.
Please cite the extracted image in APA style (or other applicable style) as follows:
Figure 2. Male holotye of Hypsiboas gladiator (Kholer et al., 2010, p. 584).
Kohler, J., Koscinski, D., Padial, J. M., Chaparro, J. C., Handford, P., Lougheed, S. C., & Riva, I. (2010). Systematics of Andean gladiator frogs of the Hypsiboas pulchellus species groug (Annuar, Hylidae). Zoologica Scripta, 39(6), 572-590. doi:10.111/j.1463-6409.2010.00448.x