Some browsers support a data URL that directly contains the data:

<img src="data:image/png;base64,iVBORw0KGgoAAAANS
UhEUgAAAFwAAAAWCAIAAADCTmwFAAAA3ElEQVR4nO2YwQ3DIAxFfysGKdM
wRmfLGJ2GjtJDqhyKAzYBB1K/UxA2Nh/HSNxifAEA4JcAID6/w3/mfnYCI
3KOKH4Ja2GOiVUKgYlCYKIQmCgE7UXp10TV2jMhCif24NfHQZpVypVkYol
St2FNr1buK9ZoCUwUgpwoU3fcI4lNUCkKZ/Pj7h4+AHhH1ouByDj1St05U
6IFOcbFoG4bby8pmQ+RceZpRjQlWpBjXAzq9ux6kB6ptOgqvDLGe/moiiK
Cs/M6TYu4TutK0UyjGGuC20cf2e+j0AJG4AOYG4/WPzawWgAAAABJRU5Er
kJggg==" alt="sparkline" />
[download]

See this random page for the code I stole.

If this is for an inhouse application where your users will be using Mozilla or Safari, this is a good solution; apparently it doesn't work in IE, though, which is kind ofa downside. :)