Mobile First approach using HTML and CSS

Thanks for looking at my solution!

I Have some questions and would be very thankful,if you could help me out.

I am not sure if I understood the concept of responsiv web-design. I tried to make the images shrink and grow at some rate until they reached their max-width/min-width. Is it a good way too handle it like that or should i set a fixed width on them? Some people used the fixed approach on their solutions, thats why im confused, which of them is right or wrong.

Another question:If i have a width of under 300 px it starts too look a bit clunky.Is that the point where i should use another media query or what is the general approach on these kind of things.