STK-BLENDER

[Alayan]

I don't know how to change the size of an "attachment" image.

The trick is pretty simple:
(1) Retrieve the URL of the attachment, for example: https://forum.supertuxkart.net/attachment.php?aid=136
(2) Use the insert image option and put the attachment's URL in.
(3) (Optionally) Specify a width/height

The attachment id are not some secret, it's just a sequentially increasing number and once you upload your attachment you can easily see the attachment's number, so you don't even really need to copy the attachment's URL, you can easily get it by changing the attachment id at the end of the base URL that could be bookmarked.