"Image Size" usually means two things:

1. First is the dimensions of the image such as 150 x 100 pixels.
2. Then there is the file size of the image in bytes.

Image Dimensions

In this article, I only discuss images that you will add into your web page content.

Use a Good Image Editor

I prefer using Adobe Photoshop Elements because it is powerful enough to edit most images and costs less than $100.

Photoshop as a great feature to "Save for the web" that you can use to set the image quality, size, etc. And it shows you how many seconds it will take for someone to download the image file.

There are others you can try but I find Photoshop works best for me.

Step 1: Prepare the image

1. Start out with a high-quality picture if you can. Then use
   some kind of picture editing software (Photoshop, Paint, etc.) to crop, highlight, etc.
2. Change the picture's size so it is no larger than 640x640 pixels.
3. Save the picture as either JPEG, GIF or PNG format.

Step 2: Upload image

1. Login to the site and click on "Create Content" link.
2. Click on "Image" link.
3. Enter a Title in the form field.
4. Add the picture to a Gallery.

Adding pictures into your content is fairly easy, especially if you follow the steps below. The process involves uploading a picture and linking to it within the content text.