In the first case, youāve got your @media blocks inside-out with your CSS. The CSS styles go inside the @media section, not the other way around. So re-organize the lower half of your CSS like this:
Regarding the question about images, I canāt tell what the actual image file names look like on the server, but Iāll bet thereās a disconnect somewhere with the name in the link, with all the various upper and lower case letters, and the actual spelling of those file names on the server. List out the file names on the server and compare very closely to make sure there isnāt a typo (suggestion: use all the same case, donāt use spaces or other weird letters, or youāll continue to have this problem in the future).
What it says on the tin. Youāve got a </a>after your headerās h1 tag (line 21) that doesnāt match anything.
All but one of these 404ād. Did you upload the pictures to the server? Do the filenames actually have underscores, or is that your web editor trying to be clever with open and close parentheses?
Thatās a mystery to me. If I copy your css directly, either into a file to validate or pasting it directly into the validator, it validates fine with no errors or warnings. So Iām not sure what the issue is. Maybe cut/paste it a second file, just to see if thereās something funky in that file thatās causing a problem with the validator?