iShot screenshot app has a very useful feature, pining screenshots to desktop. With iShot, you can pin several screenshots to your Mac screen so you can refer to them quickly without the need to switch back and forth among various apps, windows or pages.
Launch iShot, press Option + A to select the area you like to pin to your desktop. The toolbar displays below the selection box. Choose PIN icon from the toolbar to pin the screenshot to your Mac desktop.
Drag and drop it to place it anywhere on your screen. Pinch out or in your two fingers on the trackpad to zoom in or out the screenshot on your Mac screen. Right click on the screenshot to show more options and controls, such as copy, paste, save as, annotate, resize, rotate, flip, close, etc.