I'd put them all in a "Resources" folder or similar inside your project, add a Project Settings file ("app.config") which has the name of that folder, and then refer to them using that -
In your app.config file:
<setting name="DocumentDirectory" serializeAs="String">
<value>F:\Online_signup1\Resources</value>
</setting>
Somewhere during program startup:
string docDir = Settings.Default.DocumentDirectory;
Everywhere else:
string filename = Path.Combine(docDir, myfile)
That way, if you ever distribute your program / move anything around, you won't be stuck going through your code and changing filenames everywhere, just change the directory in the settings file and you're golden.
For web projects, you can put it in the web.config file, like so:
<appSettings>
<add key="DocumentDirectory" value="F:\Online_signup1\Resources"/>
</appSettings>
And access via:
string docDir = ConfigurationManager.AppSettings["DocumentDirectory"];