In my previous post, I wrote you about image formats recommended for web and e-mail and the recommended sizes for a faster download. Allright, but image dimensions will also affect to your file size, so take care abut it.
You won’t need a 3000px width and 2mb image in your web (yes, I know that your new camera can take even bigger images, but believe me, you won’t need it!!!) A 200px width is enough for a small picture enclosed in your article.
Always change the dimensions with an image editing software as Photoshop, Fireworks, Photopaint or the Gimp (this one is Open Source and free!), but don’t change it in your html editor (as Dreamweaver) because you will be changing just the representation size, not the file size.
In this image you can see some sizes in pixels over a real size grid to use as a guideline:
Bonus: Tools to measure images in your browser