Hi,
In your custom.css (line 77) you have for some reason the following CSS:
.hikashop_product_checkout_cart_image {
height: 100px !important;
width: 100px !important;
}
It seems to conflict with a generic "max-width: 100%;" for "img" in your bootstrap.css.
It's all not ideal, but if you want to stick with what's now in your custom.css, you should add there a max-width property for this image to override what's coming from bootstrap.css, so that the entire rule reads like this:
.hikashop_product_checkout_cart_image {
height: 100px !important;
width: 100px !important;
max-width: 100px !important;
}