Also, when you have the image open you can tell how big it is (if it's in accordance to the size the forum accepts) by clicking on "Image" on the upper toolbar and choosing "Attributes." It'll tell you the size of the image in pixels, cm, or inches (choose pixels). The size for pixels for the forum is 620 x 280 for bmp images, and for jpgs...I'm checking... I think there were guidelines on the old forum (something like <700 x <700?), but in the images manager it only has a total KB size: it says 244.1 for jpeg, and and 390.6 for jpg images (I didn't know there was a distinction before!).
Also with Paint, you can choose just part of the picture to show, cropping out a small part to focus on. Then you don't have to reduce in size so much the part you want to show to the forum. It's a little more complicated, once you have this resizing business down, you can learn that.