Image Border

Yes, adjust the corner radius to apply rounded borders.

Yes, borders can be added to transparent PNG images while preserving transparency.

Instagram's feed crops anything outside the 1:1, 4:5 and 1.91:1 range. Adding a white or colored border until the whole canvas becomes a square (1:1) or a portrait (4:5) lets your original photo sit inside it untouched. For panoramas or very tall vertical shots, where the aspect ratio is far off, this is effectively the only way to keep the whole frame.

Two to five percent of the photo's short side works well — roughly 20 to 50 px on a 1080 px square. In that range the picture looks tidy without the margin stealing attention from the subject. For a framed look, the traditional trick of making the bottom margin slightly wider reads as more settled. If you are posting several images together, use the same thickness on all of them so the set holds together.