Make a contact sheet or grid

Fitting scales each image until the whole of it is inside its cell, which leaves gaps around anything whose shape does not match - nothing is lost, but the grid is less tidy. Filling scales until the cell is covered and crops what hangs over, taken from the centre, which gives a uniform grid at the cost of the edges of every image. For reviewing photographs, fit. For a tidy collage, fill.

As many as your images need for the number of columns you chose - the canvas is sized from the count, so nine images in four columns is three rows and not four with an empty one. The last row is left-aligned rather than centred, which is what makes a contact sheet readable in reading order.

Yes, with the cell size set to your frame size, no gap and fill selected so every cell is exactly the same. Be aware that this exports the sheet as an image; it does not currently write the JSON atlas some engines expect alongside it, so you would need to work out the frame coordinates from the grid you chose.

No. Every image is decoded and drawn onto one canvas by your browser, on your device, and the sheet is read back out of it. Your photos never leave the tab.