If a graphic is needed to understand the content or functionality of the webpage, then it should be perceivable for people with low vision or other impairments. Enough contrast should be provided for graphics that convey important information so people with moderately low vision can perceive them.
What to Consider
Resize images at the server, if they have an intrinsic size.
Do not use images that cannot be rendered by the device. Avoid large or high-resolution images except where critical information would otherwise be lost.
Images Specify Size
Specify the size of images in markup, if they have an intrinsic size.
Provide a text equivalent for every non-text element.
Ensure that foreground and background color combinations provide sufficient contrast.
Background Image Readability
When using background images make sure that content remains readable on the device.
Do not use pixel measures and do not use absolute units in markup language attribute values and style sheet property values.
Mobile Accessibility: How WCAG 2.0 and Other W3C/WAI Guidelines Apply to Mobile
Mobile Accessibility Standards and Guidelines at W3C
W3C - The Web and Mobile Devices