If you include the header you need to link the library

In Allegro 5, each addon is its own library.
Link the image library to your project.

Adding Allegro5 with NuGet doesn't automatically link any addon libraries. You still have to link them yourself.

If you use added Allegro with NuGet it's easy:
1. Open Project Properties
2. Select the Allegro tab
3. Pick the addons you want (Select "yes" for each)

if not then:

1. Open Project Properties
2. Linker tab
3. Add each addon library to the "additional dependencies" line

Now it should be properly linked.

It should have worked

An idea: Did you do it for the configuration you are using?

Sometimes I accidentally modify properties for the release configuration when I'm working on the debug configuration.

Somehow it's not linking. for the time being, you can add the library file name to the additional dependencies.

Try that and see if it works

I'll check when I get home, but can't think of what it could be.

[EDIT]
Everything checks out.

1. create empty project
2. manage nuget and add allegro5
3. open properties->allegro5 and select addons
4. create a main function
5. compile

Here's my code:

#include <allegro5/allegro.h>
#include <allegro5/allegro_image.h>

int main()
{
  al_init();
  al_init_image_addon();

  return 0;
}

No issues.

As I asked before, when you selected the addons, did you select them for all configurations?

In this image, I only set the addon for the release configuration.

{"name":"613229","src":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/d\/0\/d0968f2b9b1056e6b0509fb7389a9d37.png","w":617,"h":428,"tn":"\/\/djungxnpq2nug.cloudfront.net\/image\/cache\/d\/0\/d0968f2b9b1056e6b0509fb7389a9d37"}
I got the same linking error because I'm not compiling in release mode.

That's the only thing I can think you're doing.

Anyone else have ideas?