Bootstrap won't resize your images. It is a CSS framework, it can only add styles to your web page but you can't use it to do any backend work, like thumbnails creation. Setting the dimensions in the HTML img tag doesn't resize your image neither, it is still the same image that is sent to the browser. So in order to boost your website performance, you need to think of another solution, you don't want to send huge images through the network if you're only gonna show them with thumbnail size.
What kind of website are you running?
For example, Wordpress automatically handles the thumbnails creation when you upload files through the Media Manager. I'm pretty sure most of the CMS have this functionnality.
If you have a PHP website without a CMS, you could try using a library such as phpthumb, that would require some coding though.
If you have a static HTML website, then you have no other choice but to resize the images manually in Photoshop.
By the way, do you optimize your images too? (compression with tools such as tinypng or jpeg-optimizer