How does alignment work if images have different sizes?

For horizontal merge: 'Top' aligns images to the top edge of the canvas, 'Middle' centers them vertically, and 'Bottom' aligns to the bottom. For vertical merge: 'Left' aligns to the left edge, 'Center' centers horizontally, and 'Right' aligns to the right.

What does 'Make images uniform before merging' do?

If checked, for horizontal merge, all images are resized to match the height of the shortest image in your selection (width adjusts proportionally). For vertical merge, all images are resized to match the width of the narrowest image (height adjusts proportionally). This helps create a more balanced composition.

How does background color work with transparency and spacing?

If 'Transparent Background' is unchecked, the chosen background color will fill any empty canvas areas created by spacing or differing image dimensions. If checked, these areas (and the spacing) will be transparent in the output PNG.