I assume you mean the background of the Skin in a forum. If so, how are you uploading it?
The recommended method now is to first upload the paper/pattern/background image into a message/post and then copy the code from the Properties.
That way, it does not appear in your Profile Image Album which has a limited capacity. If you later delete it from your album, it would disappear from your Skin.
If you uploaded it to Photobucket and used the Photobucket code and later deleted it from your album, it too would disappear from the Skin and look something like this.
The best method is NOT to click on Upload against the Background Image box in the Community Style section.
Instead, when you have copied the code from the Image's Properties, paste it into the box by doing down the Control key while you type v (known as Control v).
You can of course delete the message where you uploaded it to when you have finished, because the image will have an Aimoo code.
It depends on which Browser you are using on how you acquire the code.
Right click on the image and then left click on Properties.
Drag your mouse from the very beginning (top left) of the URL to the very end (bottom right) until the whole URL is selected. You can only see two lines at a time so my top line is not visible in the screenshot but I started from in front of the URL, dragged down and to the far right, until it was all selected.
Now Control C to copy. (Hold down the Control key while you type c).
Click on the Cancel button to close the File Properties.
Right click on the image then left click on the "Copy URL" link and that copies the code for you.
Click on the Close button to close the File Properties.
Right click on the Image then left lift on Copy Image URL.
To complete ~
In the appropriate area of your Skin settings, Control v to paste. (Hold the Control key down while you type v).
Even where it says "Upload" in your Skin settings, do not do click on the Upload button as it will add the image to your Profile Album and your 5 MB storage space would be quickly used up.
Paste in the code instead (Control v).
Click Submit at the top or bottom of the page to update your Skin.
Of course, you need to be in a Diamond forum to upload your background image into a post. If you don't have access to a Diamond forum where you can do this, send a PM to yourself with the image uploaded into it.
Open the received PM and left click on the image, left click on More then left click on Copy Image URL.
That copies the Code for you which you can then paste into the box (Control v).
I hope this helps.