You can put them anywhere beneath /Data, it is up to you if you want to have one top level folder for all your custom stuff or multiple custom folders beneath standard folders like /Data/SiteImages, and /Data/style

you could have a folder like /Data/mymoduleassets/ with sub folders as needed, it is really a matter of personal preference. Or they could go directly beneath your module folder, they don't really have to be below /Data as long as your feature knows where to find them and you configure style.config to include your css as needed.

Hope it helps,

Joe

If you are distributing a feature that needs css you should provide the css and needed images in your package so they land in a known location and you should include instructions for the end user to modify the style.config of his skin to include your css file. 

To some extent depending on the feature you may not need addtional css but can re-use known css class names already in use in your feature. As soon as you need something custom then you must provide it with instructions for the user.

Assuming the HelloWorld folder is directly beneath the root of the site

Page.ResolveUrl("~/HellowWorld/img/myimage.png");

You are missing the = like <%=, and then it would need single quotes like '<%= %>' since double quotes are in the ResolveUrl function but really you should not add javascript directly in your control. Do it like this from code:

Page.ClientScript.RegisterClientScriptBlock(typeof(Page),
                    "noblecount", "\n<script src=\""
                    + Page.ResolveUrl("~/HelloWorld/js/jquery.NobleCount.min.js") + "\" type=\"text/javascript\" ></script>");

Hope it helps,

Joe

either in code behind or if not using a code behind file inside the <script runat="server"> in page load or onint.

Doing it this way the script will only be added 1 time even if there is more than one instance of your feature on the same page. Or even if loaded by other features or controls as long as the same key is used for the script ("noblecount" in the example) is used everywhere it is loaded from.