the max size of a jpg to attach to a post is 19.5 KB and 650x650 resolution
there are many sites on the web that you can resize and crop pictures online...just google for "free image resizer" and you get results like picresize.com
what i do is use a program called gimp (
GIMP - The GNU Image Manipulation Program) which is a free photoshop alternative to resize photos. just open your photo and go to image and scale image and put in your desired X and Y (if the 'chain' is together when you change the X and click out it will change the Y to keep the aspect ratio). then you just play with it to get the size and resolution you want.