Force WP to keep the image size same

I'm using Phogra theme for my images. I've uploaded the stuff, created pages(sub-pages), every thing seems to be working fine, but when I click on some Album(HERE, let's say on Wedding OR Events), that album get loaded to Slider(obvious), but I've images of different sizes for different images, so when I click on album, some images get cropped automatically, and some are stretched. I really want to keep the images in their original size.

WP version Version 4.3.1.

Thanks in advance for your help.

via Chebli Mohamed