Custom screen savers for a mobile phone can be made with Microsoft Paint. Start by finding the phone's screen resolution; resolutions for almost any model of phone are found at Cartoonized.net. Select the correct cellphone brand, and the site displays a list of models with resolutions.
Open Microsoft Paint, then open the image to be used as a screen saver. Click the Resize button in the Image section. For the moment, uncheck Maintain Aspect Ratio. Select the Pixels radio button.
In the text box next to Horizontal, enter the first number of the screen resolution. In the text box next to Vertical, enter the second number of the screen resolution. Do not type anything into the two text boxes under Skew. Click Okay, and look at the results. If satisfied, save the image as JPEG, and upload to the phone for use.
If making a screen saver from scratch feels like too much, there are sites such as RedDodo.com that provide screen savers the user can customize with his own text. Many of these sites provide some screen savers for free. Others are available for a fee. Use common sense, and be careful to avoid viruses and spyware when downloading anything onto a mobile phone from a third-party website.