When you’re done with your Photoshop files and want to upload them online, press Cmd/Ctrl+Opt/Alt+Shift+S (File>Save for Web)
In the Save for Web menu, for images set them to JPEG, and set Quality to 80. Tick Optimized.
Make sure Convert to sRGB (the colour dynamics of most displays) is ticked and Preview is set to ‘Internet Standard RGB (No Color Management)’.
If your image is big, you may need to lower its pixel dimensions. About 800px wide should suffice. Use the fields under Image Size to make these changes.
Save for Web flattens your image (keeping the layered version open), and makes it web-ready, meaning it will look good on most screens, and uploads quickly.