Typo3 sr_feuser_register e-mail template not working img tag

i have to change the default confirmation emial template in sr_feuser_register ext, i'd changed succesfully every markers content but i have to insert an image into the html email and it don't want to work... i tried to replace the marker with te whole i.m.g. tag (img src="....") and i've tried ti replace only the src (img src="marker")... but dosn't work. anybody met this problem already?

Please enable html:

plugin.tx_srfeuserregister_pi1.create.fields = module_sys_dmail_html .....

plugin.tx_srfeuserregister_pi1.create.defaultValues.module_sys_dmail_html = 1

and in template file tx_srfeuserregister_pi1_css_tmpl.html
<!-- ###TEMPLATE_CREATE### begin -->
...........
<!-- ###SUB_INCLUDED_FIELD_module_sys_dmail_html### -->
<dt>

</dt>
<dd>
<input type="hidden" title="" name="FE[fe_users][module_sys_dmail_html]" value="test">
</dd>
<!-- ###SUB_INCLUDED_FIELD_module_sys_dmail_html### -->
.............
<!-- ###TEMPLATE_CREATE### end -->

Flexo's answer did not work for me. Instead, I activated Direct Mail and set the dmail-Checkbox in the HTML-Template to hidden.

<!-- ###SUB_INCLUDED_FIELD_module_sys_dmail_html### -->
...
<div style="display:none;"><!--TCA_INPUT_module_sys_dmail_html--></div>
...
<!-- ###SUB_INCLUDED_FIELD_module_sys_dmail_html### -->

Also, in the Constants you can set

plugin.tx_srfeuserregister_pi1.dmailCheckedUponRegistration = 1

to make sure the checkbox is checked on default.