If you have good quality images and want to use them in your wordpress website they most likely will take a lot of space on your hosting account. They would also slow down your website performance. Visitors would have to wait until it fully loads. 1 or 2 pictures wouldn’t be a big issue but imagine the whole gallery of huge images! Every image before it goes to a website should be resized and optimised. You could do it in any image editing software like Adobe Photoshop or just use one of the plugins available for wordpress.
I would recommend using Imsanity as it works automatically whenever images are uploaded through wordpress CMS.
Imsanity settings allow you to change quality (compression) and maximal size of the image.