First of all, whatever custom icon you upload will be resized to a 100x100 pixel square. It will first scale the smaller axis (width or height) to 100 pixels, then cut the larger axis to 100 pixels. So for example, if you have a portrait-shaped image it will trim the top and bottom, and if you have a landscape-shaped image it will trim the left and right.
The icon image will then be scaled to fit the location on the card. The large icons at the corners are 100x100 px, the smaller icons at the left and right sides are 60x60 px, and the inline icons are 30 px.
All of the icon locations themselves are in a fixed position and cannot be moved (at least for now).
So... if you want to make sure your custom icons are in alignment, just create them on a square canvas of any size. Then when they are uploaded to the app, they will be scaled appropriately based on where you put them.
For the BBCode on icons, let me do some testing and see how the user experience is entering them on those small text boxes. I had considered adding this functionality before, so I'll take another look now. Thank you for the suggestion!