@wbmnfkt - yes I'm new to webdev and finding my way around it all. I'll look through more of those tutorials and I've watched a good number of Lahijani's videos which are very good. I'm building my website gradually and figuring stuff out as I go along. I need to read more tutorials on HTML, CSS and PHP but learning while doing is the best approach for me.
@ Chris - I'll use devtool more but it's a bit confusing at first and need to figure it all out. I'm just puzzled as to why the problem with resizing went away once the main.css file was named anything but that name. My header image resizes fine now and is not limited to 100px width.