In order to send a message in HTML format with online pictures you must have those pictures available somewhere on the Internet, usually a folder on your website. In our case we have a directory at maxprog.com where we store all our pictures.
For referencing your pictures inside your HTML code you have the choice between using a 'base href' header tag and the relative address of your pictures or just using absolute addresses everywhere. Remember that an absolute address (or URL) is the one that includes all the information about the location of the file it references including the protocol (http:// or https://), the full domain name (www.mydomain.com) and the file path. For example www.maxprog.com/pictures/mail.gif is an absolute address. An example of a relative address could be 'pictures/mail.gif' or '../pictures/mail.gif'. To make things clear, an absolute address is the one you could perfectly use in your browser to access a file. A relative address will never work on a browser since half the address is missing! An absolute address always works, a relative address will only work if you have added a 'base href' tag to the HTML code header part. Be careful, an address without the protocol (http:// or https://) like 'www.maxprog.com/pictures/mail.gif' will never work!
So if your HTML code uses relative addresses to reference your pictures and you are too lazy to convert them into absolutes you can use a 'base href' tag in the header part of your HTML code. This is easy to do but it has a great drawback, it will not work everywhere, especially given webmail systems like hotmail will simply ignore that tag and none of your pictures will get displayed. To add that tag just locate the head part of your code, it is the text between the <head> and the </head> tags. Just add your 'base href' tag line this way: <base href="http://www.myweb.com/">
Then to reference a "picture" named picture1.jpg from folder "pictures" into your HTML code, the code would be <img src="pictures/picture1.jpg"> You got it, when rendering the HTML code the recipient software will add the 'base href' address to the picture address. This is why such addresses are called 'relative', they are relative to the base address, in this case the address set by the 'base href' tag. If you try to preview your document locally, it works perfect. If you send your message the recipient will simply get 'picture1' from: http://www.myweb.com/pictures/picture1.jpg well, but not with Hotmail.
As said before this method, despite being easy-to-use and straightforward has a great drawback, it will fail in webmail systems like Hotmail. Indeed Hotmail will simply ignore the 'base href' tag and none of your pictures will get displayed. In such case the only alternative is to use absolute addresses everywhere in your code, that is, addresses fully referencing your pictures on your site. For example if your HTML code contains the reference <img src="pictures/picture1.jpg"> you have to complete it with the protocol and the full domain name: <img src="http://www.myweb.com/pictures/picture1.jpg">. Now you can be sure every single recipient in your list will be able to see your picture. It is a bit more work since you have to check all addresses one by one but the result is well worth the trouble.
My recommendation, don't be lazy and make all your addresses absolutes!