Open your pic with Paint, use the Image tab, then click Stretch/Skew... enter about 40% or so and click OK.... click on File and "save as" , then use that pic to post with. You now have two copies of your pic.. the original, and one small enough to post with. You may need to play with the percentages a bit until you get it how you want.
Preview your post... what you see will be what we see.
Or you can try the Image re-sizer here........ (I could never seem to work it correctly so I use the other method)
PM me if you get stuck, and I will walk you through it.
Kevin