As you already know most of the current e-mail clients (either webmail or desktop) will disable the images in your e-mail by default. Unless your recipients decide to show images or to add you to their white list, images will not show in the screen.

This is a challenge for e-mail marketing design, because you must take this into account and create designs that work well even if images are not shown.

A quick way to test your creativity when images are turned off is to use a couple of web developer tools. In fact they are very similar, but you must choose one or the other depending on what is your browser of choice (Firefox or IE). This tools are very powerful and a must-have for every web developer. They both have a lot of features that I'm not going to describe now, but take my word for it and give them a try. I'll stick to the couple of things that will help you to test your designs.

If you're using Internet Explorer there is a tool called IE Developer Toolbar that you can download from here. When installed you it will show a small arrow icon in IE's toolbar:

If you click on it a small aditional window will appear at the bottom of IE's window with a lot of options. Search for the "Images" menu as shown:

If you disable images you will have an immediate feel of how your design will be shown in an email client with images turned off. You can even modify your HTML directly in the tool's window and see changes in real time.

A very similar tool is available for Firefox, and it is even easier to use. It's called "Web Developer" and you can download and install it from here. Once installed you get a wide toolbar just above the current webpage with a plethora of menus and buttons aimed at the web developer. The equivalent feature in this tool is located under the "Images" menu:

This has been just a quick tip on how to test your designs without images, but if you have HTML/CSS working knowledge and give both tools a try, as soon as you start to scratch the surface you will find fantastic features that will easy your work a lot.