Greetings Gurus 
Recently I have requested one of my friend to design a website for a non profit charitable organization. My friend developed using html and bootstrap. He delivered the website. I found the website is loading slow.
The website has full page image with slider in homepage. All together 3 images which can be changed by using slider. The image size, resolution may be the issue.
Please refer to the below image for image size details
[ATTACH]66560[/ATTACH]
My friend is sick now and hospitalised. So it is not a good time for me to ask his help to modify images. I have photo shop. I need your help or share some links so that I can do it myself. Here are the area I need help.
1) As per the image, all image sizes are 1900 x 920 pixels. The resolutions are 300 and 72 dpi. Given the current Desktop/Laptop Displays resolution and cellular phone, what resolution I should save, what should be the size?
2) If I need higher resolution (as one of my image is 72 dpi) Can I simply increase the resolution to higher resolution in Photoshop? The images are natural agricultural farm.
3) How do I reduce the file size without affecting the image dimensions (width x height)? Any guidance? Tutorial links? Appreciate
4) I would like to replace one image s2.jpg with another image which I took using my Digital Camera. The replacement image size details as follows
[ATTACH]66561[/ATTACH]
What should I do in Photoshop to optimize the image?
Thank you and appreciate your inputs
