If you want to use "Read more" that you don't have to add <span class="fullpost">.You will be in difficult with fixing your entries, it can be crashed because of "Read more".
So now I will lead you how to create auto "read more" supports thumbnail.
With this version ( I will lead you to create auto "read more" with no thumbnail in another post), it will take the 1st picture in your post to appear in homepage.This is from vietwebguide.com,that place you can view the full source but I hope you understand VietNamese.
Go to Layout--->Edit HTML----> Expand widget templates, find this code
<data:post.body>and change it by this code instead:
<b:if cond='data:blog.pageType != "item"'>
<div expr:id='"summary" + data:post.id'><data:post.body/></div>
<span class='rmlink' style='float:right'><a expr:href='data:post.url'>-->Read more...</a></span>
<b:if cond='data:blog.pageType == "item"'><data:post.body/></b:if>
Then you have to add this code before tag </head> :
summary_noimg = 430;
summary_img = 340;
img_thumb_height = 100;
img_thumb_width = 120;
Notice the red code:
summary_noimg :the characters summary describe your entries that dont have image.summary_img :the characters summary describe your entries that have image
img_thumb_height : image height (pixel)
img_thumb_width :image width(pixel)
When done , your entries will look like this:
Or you can view another simple "Read more" form, click here for instructions.
Well if you have any problem, please contact me.
thank you and good luck