You could try the data URI scheme. It allows you to encode an image file directly into your HTML code.
<img src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4//8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU5ErkJggg==" alt="Red dot" />
That example is taken from this Wikipedia article, which also provides a PHP example. It's very easy to do.
The caveat to this is browser support but it sounds like most modern browsers, including mobile, should support it. Give it a go and see, should take no time to implement and you'll know right away if it's the right solution.