6 Answers, 1 is accepted

I just open the ticket to show my solution.
Whould be great if it had a property ImgUrl, but, here is my code.
<style type="text/css">
.picCheckBox { display:block; }
.picOfCheckBox { position: fixed; margin-top: -26px; margin-left: 18px;}
</style>
<div style="display: block; width: 100%;">
<telerik:RadCheckBox Checked="false" CssClass="picCheckBox" ID="chk1" runat="server" Text="Some text" />
<img class="picOfCheckBox" title="opo" src="16.png" alt="opnc" width="16" height="16" />
</div>
<div class="clear"></div>

The most likely reason for this is that there is another Telerik button on the page with a different RenderMode: https://docs.telerik.com/devtools/aspnet-ajax/controls/checkbox/troubleshooting/distorted-appearance.
Also, if you want to have a custom image in a checkbox, I suggest you consider a ToggleButton instead: https://docs.telerik.com/devtools/aspnet-ajax/controls/togglebutton/overview.
In case the built-in icons do not suffice for you (https://docs.telerik.com/devtools/aspnet-ajax/controls/togglebutton/functionality/Icons/embedded-icons), its toggle states expose an Image tag where you can provide a URL: https://docs.telerik.com/devtools/aspnet-ajax/controls/togglebutton/functionality/image.
Regards,
Marin Bratanov
Progress Telerik

Both solutions that you say are impossible to be made at this point of the development.
Thanks.
You can also use CSS to insert the image in the text span of the checkbox with fewer overrides:
<
style
type
=
"text/css"
>
.picCheckBox .rbText {
background: url('16.png') no-repeat no-repeat;
padding-left: 20px;
}
</
style
>
<
telerik:RadCheckBox
Checked
=
"false"
CssClass
=
"picCheckBox"
ID
=
"chk1"
runat
=
"server"
Text
=
"Some text"
/>
Regards,
Marin Bratanov
Progress Telerik

Hi Marin:
Works, but with some change:
.ChkTYPE
16
.rbText {
background-image
:
url
(
'16.png'
)
!important
;
background-repeat
:
no-repeat
; }