You can; see this for more information. Essentially, you should format it as:
<% if (DataBinder.Eval(Container.DataItem, "Voted") == "true") { %>
<img style="cursor: pointer" onclick="Dostuff"
id='<%# DataBinder.Eval(Container.DataItem, "Uid") %>'
src="../../Images/badge-circle-plus-24-ns.png" />
<img style="cursor: pointer" onclick="Dostuff"
id='<%# DataBinder.Eval(Container.DataItem, "Uid") %>'
src="../../Images/arrow-down-24-ns.png" />
<% } else { %>
<img style="cursor: pointer" onclick="Dostuff"
id='<%# DataBinder.Eval(Container.DataItem, "Uid") %>'
src="../../Images/arrow-up-24-ns.png" />
<img style="cursor: pointer" onclick="Dostuff"
id='<%# DataBinder.Eval(Container.DataItem, "Uid") %>'
src="../../Images/arrow-down-24-ns.png" />
<% } %>
I'm not sure if that's completely supported, so you could also consider trying:
<img style="cursor: pointer" onclick="Dostuff"
id='<%# DataBinder.Eval(Container.DataItem, "Uid") %>'
src='../../Images/<%# DataBinder.Eval(Container.DataItem, "Voted") == "true" ? "badge-circle-plus-24-ns.png" : "arrow-up-24-ns.png" %>' />
.
.
The latter uses a conditional and renders the appropriate image depending on the condition.