Choose a layout, drop in photos and drag each one to set what shows in its cell. Set the shape for where it's going, add a border, and save a single sharp image. Your photos stay on your device.
Social sites resize everything to a fixed width. On a 1080-pixel Instagram post, each cell of a 3 × 3 grid gets under 360 pixels across, about the size of a thumbnail. These are the pixels each cell of your current layout will actually get.
A grid of random photos reads as clutter. These small choices make it read as one picture.
Each cell is filled edge to edge, so a 4:3 photo in a tall cell loses its sides. The left panel measures exactly how much of each of your photos the current layout throws away. The right panel uses the other approach, from photo sites like Flickr: keep every photo whole and choose row breaks so the rows come out close to one target height.
A swipe-through panorama splits the collage into carousel slides; a “puzzle” profile splits it into posts that line up on your profile. The collage is re-rendered large enough for each tile to be 1080 pixels wide, then cut. The dashed frame shows the 3:4 thumbnail a profile grid displays, so you can see what falls outside it.
Common upload sizes and what each feed does to them. The “cell at 2 × 2” column is computed from your current spacing. Platform sizes come from general knowledge of their help pages and change often: check before a campaign.
Neighbouring tools, with a typical result for each.