I use Paint to change mine, you go to Image, Sketch/Skew, and just change the percentage equally in both width and heigth. I'm sure most people have a better imaging program, but I'm just lazy.
That's how I do it to..my daughter showed me how. In paint, you can check the image attributes, keep shrinking until you get it below the specs that AC will let you upload, and then you can link it.
if its less than 1megabyte http://imageshack.us/
click browse, select your image, click host it and it will give you the url which you can then put in your posts
too easy