Align to Artboard leaving a thin white line along the edge (Object is not perfectly aligned)

I created an Artboard that is 5.5" tall. I placed two images, and made each one 2.75" tall. I want to align them vertically, so that they take up the whole Y-axis of the artboard (and I assumed they would fit because 2.75 + 2.75 = 5.5").

I selected the images one by one, opened Window > Align. In the Align window, I used the "Align to Artboard" option. The image on the top seems to have aligned correctly. However, the image on the bottom is not quite at the bottom of the Artboard. There is a thin white line between the edge of the Artboard and the bottom of the image.

Does anyone know how to fix this?

My Preferences are set up as:

Grid Preferences:

• Style: Lines
• Gridline every: 1 in
• Subdivisions: 8
• "Show Pixel Grid (Above 600% Zoom)" is checked.

Smart Guides Preferences:

• "Alignment Guides" is checked
• "Object Highlighting" is checked
• "Transform Tools" is checked
• "Anchor/Path Labels" is checked
• "Measurement Labels" is checked
• "Spacing Guides" is checked
• "Construction Guides" is not checked
• Snapping Tolerance: At first it was set to 2 pt, and then I tried using the Align to Artboard feature with Snapping Tolerance at 0 pt, but still have the same white line at the bottom.

Thank you in advance for any help/suggestions!