As Adriaan suggested, you want to change the quality of the picture not just the physical size. I don't think the Paint program that comes with Windows will do that; but plenty of other picture editing programs do. I use IrfanView; it's free, light weight, easy, and fast. When you go to Save As, and choose jpg, you then get the option to reduce the quality of the picture. This will significantly reduce the KB size without making any changes to the height and width. And usually the reduction in quality required to get down to 150kb is not too noticeable. I've added an add-on to IrfanView that lets me automatically reduce the quality for a file size of 150kb when I save the file.
I will add however that it can be helpful to reduce the width of the picture so that the text of any other comment posted to the thread doesn't run past the width of the browser windows the other members are using to read the messages. If people are having to scroll back and forth to read your comment, then your comment isn't going to be read by as many people.
So, ideally, first reduce the picture size to a reasonable width; then reduce the quality to finally get to the 150kb limit.