Well, hmmm. Do you have some digital photo handling software
on your computer that you use to manipulate your photos? If you dont, then you probably can use whatever program your computer uses when you double click your pix to open it. If its a basic Windows setup, it will probably open your photo in "Paint". You can use that to resize your photo:
Open your picture in Paint
Click on the "Image" in the menu,
Click on "Attributes"
Under Units, click on the unit of measurement you want to use to determine the size, its probably pixels.( if you look at the allowable file sizes on the Manage Attachments menu in the forum post it will tell you the max size you can upload.)
Type the measurements in "width" and "height"
Save your modified photo with a new file name!! This is important, because if you just use Save instead of Save As, then you will overwright your original photo with a smaller version, which will lose resolution. So give the reduced one a new name, I usually just add an A to the existing name so I know its been messed with.
Then use the other steps to insert THAT saved smaller image to your post.
That should work.