First of all, by resize, I mean - physical resize, when image is stretched on the fly with thumbnail (or WITH copied thumbnail). Width and height in HTML, by the same time, do not physically resize picture on the server - they resize them by their program in memory, so every smaller picture gets resized, too. So, you may set these values by hand, but by resizing linked images on the fly on the server side, you need a complicated and not really fast algorithm. It is not implemented in miniBB, but soon, there will be a files plugin available, which will make possibility to not just make image links, but also upload images themselves. It is in testing study now.