[jQuery] like 'isset'...determining is something exists on the page

[Txt.Vaska]

I'm trying to determine if soemthing has been set on the page...and  
if it is do a different action. I have...

if ($('a#f_' + ide).siblings().is('img'))
{
  // do this action
}
else
{
  // do something else
}

Which looks to see if an image exists in

I'm expecting that .is() will return true in some cases but it never  
does.

Am I going about this entirely the wrong way?

Thanks