Step 1:
Open the image in Photoshop
Photoshop > File > Open
Choose the image

Step 2:
Go to Image > Image Size
Step 3:
This image’s original size:
Width: 4896px
Height: 2752px
Resolution: 350ppi
Step 4:
Change the Resolution to 72 ppi.
Once changed to 72 ppi.
The Width and Height will automatically be changed too.
Step 5:
Leave the Resolution in 72 ppi.
Change the Width back to be your exact need for the website. In this case, I need this image to be the background for the section, so I used 1280 px for the Width.

Step 6:
Now, save the file.
File > Export > Save for Web (Legacy)

Step 7:
Often PNG file is smaller than JPEG. So choose JPEG and choose Medium or High for quality. And then <SAVE>

Finally, it’s done.
We managed to change the file size from 6.4 MB to 435 KB.

Yeah! Yeah!