1. Write your article and click on 'Image' under 'Add media'.
2. Click on 'Choose your picture' and pick the image you want to upload
3. Edit the width of the picture by manually entering the number of pixels.
4. Don't worry about keeping the picture in proportion, that's done automatically for you ! All that's left for you to do is click 'Add' and voilà, your resized picture is on-line !
Tip : Your picture not getting any bigger ? It's probably because it's original size isn't big enough.