Images création for IT website

Hello,

I’m the president of a company named Synergy. We were recently in touch for another Project about designing my company logo. I want now to create several images for my website. These images will represent it domains like architecture, development, Software computing, big data, DevOps,.... Our website is the following: synergy-group.fr.
Basically we nées to change all the images you can find on the different pages on this site.
If you are interested, can you provide us an estimation of the cost? Let me know if you need more details.