You need to know two things:

The difference between relative links and absolute links

and

What happens behind the scenes in your subdomains.

The place for explanations of both questions-- often both in the same thread or even the same post-- is down the road in the Apache forum. You can try a Forums search and come up with mountains of hits, some of them sounding suspiciously similar.

Most important:
Normally subdomains involve rewriting, so the user sees "subdomain.example.com" but the page content really lives at "www.example.com/subdomain". (Other configurations are possible, but this is the most common.) Links refer to where the browser thinks the page is located. Requests for non-page material-- images, stylesheets and so on-- may or may not get rewritten along with requests for pages. Your host probably has a page of boilerplate explaining this part.

Do you yourself have to do anything in htaccess about the subdomains, or is it done by the host?

it is done by host.

For me I have to extract image by using subdomain path in an html file. Other wise the same subdomain path pulls the image file when typed in browser.