Pika

Pika free and open-source web app to quickly beautify your screenshots. You can add background gradients, tweak shadows, add rounded corners, tweak spacing and more. You can save the image as PNG file or copy it to clipboard.

Qurb lets you generate mockup for iPhone X/XS/Max, iPad, MacBook and other devices quickly. Just select the device, add your screenshot and save the mockup. All the devices are designed in CSS. Also when you add your screenshot, it's not uploaded anywhere.