If it's all the same I'd like to leave it all up for the benefit of others, and so that the comments I am about to make make sense.
A "URL" is an address, Uniform Resource Locator. For example the URL to the Erato forums is "http://www.ablemuse.com/erato"
Pictures have addresses too. This is what you need to say when calling for the image to appear in the thread. URLs happen when you put something "out there".
Now, Julie said: "In the top row of doohickeys and thingamabobs and bells and whistles, click on the icon of a paperclip. This conjures up the "Manage Attachments" menu.
Under "Upload File from Your Computer", select "Choose File", and you should see a menu of the contents of your computer. Navigate to wherever you've stored your pictures, "Select" the one you want, and voilą."...and all this is quite true. This is one way to do it.
But see what happened? It makes an attachment link. If you want the image to be a full part of the thread you would still need to click it to open it, right click the large open image, select "Copy image URL", then come back in and edit your post, type up the code with image tags, and finally delete the attachment business.
This is why I always begin by uploading my image into my albums (provided by Eratosphere).
1 - User CP tab at the top of every Eratosphere page (User Control Panel) right below the Eratosphere and Ablemuse logos.
2 - On the left side about 5 items down "Pictures & Albums".
3 - Then select an album or "Add Album".
4 - "Upload Pictures"
5 - After it's uploaded, open it. Erato automatically has the "image URL" and the "BB code" all ready there for you to copy. The BB code is the whole thing with image tags. Just copy that and paste it in your thread, easy as pie.
As for size - 700 pixels wide is a nice size for these forums that won't stretch the page width, forcing people to scroll sideways. I tried out your image and it's kind of big.