User839733648 posted
Hi dlchase,
According to your description and code, I could not reproduce your problem.
I've made a sample and the following code is similar to yours. My image works well.
<asp:ImageButton ID="BtnSubmit" runat="server" Text="Submit Form" OnClientClick="return reCaptcheck();" ImageUrl="test.jpg"/>
result:

In my opinion, maybe there're some other codes making the effect.
If possible, please provide more details about your page so that it will be easier for us to help with you.
Or just as mgebhard has said, you could use F12 develper tool to check if there's an error and to solve it.
Best Regards,
Jenifer