I'm trying to figure out how to close the ends of my tower together. I'm new to free building. I used round studs and 2×1 bricks. Can anyone help, or did I make a mistake somewhere?

Do you have a picture?  I'm having a hard time visualizing what you're describing.

I posted images...they didn't seem to upload. Let me try again.

I posted images...they didn't seem to upload. Let me try again.

It says I've used up all my attachment space by posting my octagon question, but won't let me delete it

ok, I deleted the image, but its still taking up 24% of my attachment capacity. Any idea how to fix that?

https://500px.com/photo/1013907886/20200412084327-by-Allysa-Hill

https://500px.com/photo/1013907885/20200412084350-by-Allysa-Hill

 

Ok, let's see if this works.

The direct upload to Eurobricks is for your avatar only, which is why the space is so small.  You should be able to re-load another picture for your avatar to get rid of the other.  All the other pics should be hosted on an external site, like the one you have.  If you put the image BB code brackets around your link, they will show up in-line, like the example below:

https://www.bbcode.org/examples/?id=10

As for closing the loop, just continue the 1x2 and 1x1 round stud on the loop, and it will finish itself.  An example is done with bricks instead of plates (but same idea) is below:

 

Thanks, alot. I really appreciate it.