How To Compress Images For Web

Open the image in Photoshop or image editing program
Save for Web (see notes below for Photoshop)
To save for web: File > Export > Save For Web (Legacy)
Keyboard shortcut: Alt + Shift + Ctrl + S 
Check settings for web (see notes below)
Settings Checklist

File Format: JPEG
Quality/Compression: Usually Between 50 & 75
Progressive: CHECK
Embed Color Profile: CHECK
Convert to sRGB: CHECK
Metadata: None
Image Size: 1500px wide (height will auto-adjust)

Save the file - Be sure that the file is no larger than 1900 wide. The larger the image the more space, and time it will take to load when used on your site.