Capturing high resolution screenshot

Hello! 
I need to insert a cropped screenshot into a slide and maintain a 200 kb resolution for the cropped photo. When I take the initial screenshot it is 225 kb, but when I crop it to what I need it to be, the highest I can get is 58.4 as a PNG file. This is when I crop after using the PrtScr button. 
The required Storyline screen size is 960x600. 
I've tried taking screenshots using the built-in Storyline screenshot feature and the size is around 18 kb once I crop. I've also tried the Windows Snipping tool but it only gets me to around 20 kb. 
Any suggestions or thoughts are appreciated- thanks so much!