The same concept applies for browsers as for mobile devices. Handle multiple viewports for mobile for visual testing – Imagium Technical Discussion – Imagium Forum

Before proceeding to a solution, lets understand how Imagium works. In Imagium any snapshot inside a project is uniquely identified with Test Name and Step Name. Playing around with them gives you flexibility to deal with other similar scenario.

You have two options:

• Create Test Name at runtime
• Or create Step Name at runtime

If you want to distinguish between the resolution, you can simple change the set the TestName/StepName at  runtime like :

Login640X480,  Login1920X1080 (Imagium will automatically create two versions of the baseline)

		int width = 1920;
		int height = 1080;
		driver.manage().window().setSize(new Dimension(width, height));
		String uid = getUID("Test Color"+width+"X"+height, "b84eb8f0-3056-4f3c-9a4e-5e8cfd3e33f7");