Resizing Images if they're Too Large
In most cases, you can resize images by clicking and dragging a corner of the picture in the admin area, or by right clicking the image, going into the properties and entering in new size.
However, if the original image is very large (for example, if you've taken a photo with your digital camera) it's best practice to reduce the size of the image before you upload it to use on your website.
To do this, you can go to http://www.shrinkpictures.com. There, basically you'll have a form that will allow you to upload a large image and make it into a smaller one.
It's all pretty self explanatory. Question 2 on the page, if the picture is for a gallery/album you want to resize it to 600 pixels, if it's for your website 350 pixels will do. If your images are quite large so you might have to wait a while for the website to process your images.
If you'd rather download a program to do the same thing, you can get one at http://www.vso-software.fr/products/image_resizer/