Completing using SASS and BEM

This was my second challenge in the Junior category. Took a lot of time to complete this challenge but was surely a learning experience. I had faced a few issues on this one:

1. At times the styling on one element can drastically affect the elements around it, which becomes difficult to handle, especially when trying to make the page responsive.

2. what would be the best way to make the images responsive and still not let them affect the page's layout? on this project, the images sometimes take absurd dimensions which would affect the layout of my page.

3. Lastly, right now the login and signup button on the top right keeps moving down when I open my menu on the left. I tried setting it to float: right, but it doesn't work.

Also, I feel I need to take a modular approach to each section of the page so that my final sass file is not too big. Are there any models that I can follow to implement the design in a modular manner?

Any feedback would be appreciated.