Why the posting order is reversed
Your profile shows the newest post at the top left. To make a big picture appear in one piece, you post the bottom right tile first and the top left tile last. The files are numbered for exactly that: post the file starting with 01 first, then 02 and so on. The preview shows the same numbers on each tile, and the file name also keeps the row and column (for example photo_01_r3_c3.jpg).
Choose 3 × 1 for a banner across one row, 3 × 2 or 3 × 3 for a larger picture, or 3 × 4 for a tall one.
Square or portrait tiles
Since 2025 Instagram shows profile grids with taller 3:4 rectangles instead of squares, so square posts are trimmed a little at the sides in the grid view. Pick Portrait 3:4 if the profile view matters most, Square 1:1 for the classic look or Portrait 4:5, the long established tall size of feed posts. With Crop the center to fit, the picture is trimmed evenly from the edges so every tile has exactly that shape.
Honest limits: posts you add later push the grid along, and a picture only stays aligned when you add posts in multiples of three. Pinned posts are shown first and can break the picture, and Instagram may change how it crops the grid again. Check your profile after posting the first row.
Seamless carousel
For a panorama that people swipe through, choose Carousel and the number of slides. The tiles are cut side by side in reading order, so slide 01 goes first in the post. All slides of a carousel use the shape of the first one, which is why every tile here gets the same size.