since you're using imgur, you can edit them on the site itself.
just go to your images, click the little pallet and brush on the lower left corner of the
image you want to edit, then click "edit" at the top..
Then just re-size it to what you with the pixel sizes on the top right of the image
and click "apply" and "save".. (make sure to keep proportions locked)
this'll make your url link to the image end in ?1, so you just paste that in the
[IMG] tags and you're done...
On another note, you can just take off the ?1 and you'll get the full size image.