How can i edit these?

If you use the attachment function of this board and not an external image hoster, i will have a look at the image and maybe i'm able to help you then

If you want to replace specific of these icons, you can do that via ACP -> Content -> list Boards -> edit the board you want -> scroll down to the end -> specify icon for that board
make sure there is no mark in the "show as background" box.

If you want to replace them ALL, go to the style editor and definde your own icon path for your style. then simply upload an icon named "boardM.png" into that path and it will be displayed as icon for all boards.