From blueprint it is quite easy, just use a “Launch URL” node like this:
For C++ this should work:
FPlatformProcess::LaunchURL(TEXT("http://imgur.com/gallery/X4Sr3"), NULL, NULL);
Api docs here: FGenericPlatformProcess::LaunchURL | Unreal Engine Documentation
Both of those should open the page in what ever browser that the user has set as their default.