upload them to imgur, grab the link and then click the image button on here, click the hyperlink symbol, input the link. (don't forget to add .png or .jpg at the end if it isn't there)
Then, click on the picture, a box with some options should come up. Click the the lil plus sign (it has arrows pointing each way) and then set either the width or the height to the appropriate size. I find that 800 width is a good size for screenshots.
for future reference I believe that any image above like 2MB can't be uploaded with the forum uploader, you'll need to use an external host like Imgur or ImgBB