Note: if an image is purely decorative, the attribute should have a null value, and this will cause screen readers to skip the element.
Note:For context, approximately 27.6% of web users have a screen resolution of 1366x768px. And about 95% have a screen resolution of 1024x768px or higher (1280x800, 1280x1024, 1366x768, 1920x1080 or higher). When designing for these variations in screen resolution, it is helpful to target common a common resolution like 1366x768px, or use percentages.
You should be able to get size/proportion information of your image when opening it locally. You can access the dimensions of a picture by activating the applications menu (JAWS is Shift F10) and selecting "get info" on the image menu. Once you have that information, you can use the proportions to calculate a size that you feel fits in with your page. Your paragraph text default size is approximately 16px in most browsers and h1s are approximately 32px. Think about the proportion of your text content to the images you want to use.