Search should cover all the areas, but clearly and simply segregate results for Albums, Slide shows, Blog posts.
Don't make the user think about whether they need to search for one particular channel or all channels or anything else - just do it.
I know websites like Amazon for example allow you to choose from a drop down, but the difference is that on Amazon I might be searching for a book or I might be searching for a toy. Therefore it is (generally) clear which department I want. (In any case the default is all departments, and most people will stick with that).
However, here you are always searching for a single thing - a photo, and you have no idea which 'department' you might find that photo.
The detail of 'how it looks' is up to you and the design of the rest of your website. If the user is likely to end up with many results, then consider the ability to filter the results once shown - but you don't need to provide that filter up front.
Information Architecture for the World Wide Web by Peter Morville and Louis Rosenfeld says:
...look for opportunities to educate users when they’re ready to learn.
The best time to do this is after the initial search has been executed
and the user reaches a point of indecision or frustration. The initial
hope that the first try would retrieve exactly what they were looking
for has now faded. And when users are ready to revise their searches,
they’ll want to know how they can make those revisions.