Vertical or horizontal?
Vertical suits anything that reads top to bottom: a conversation, a series of steps, a long receipt photographed in parts. Horizontal suits comparisons, where the point is to see two things side by side. If you are unsure, vertical is usually safer, because phone screens are tall and a wide image ends up too small to read.
Why sizes need matching
If one photo is 4000 pixels wide and another is 600, placing them together leaves the small one looking lost in a sea of empty space. Scaling everything to a common width, or a common height when going sideways, keeps the result looking deliberate rather than accidental.
Gaps and background
A small gap between images makes clear where one ends and the next begins, which matters for screenshots that would otherwise blur into one another. A white background looks clean on light interfaces; black works better for photos and dark screenshots. Choosing a transparent background produces a PNG you can place on any colour later.
Order matters more than you think
Files rarely upload in the order you intend, because most systems sort by name rather than by when you picked them. Check the sequence before merging, particularly for anything where the order carries meaning, such as a step-by-step guide or a chronological conversation.