While editing the page's HTML code, locate the <body> tag near the top and ensure that it reads:
<body background="file.jpg">
where "file.jpg" is the name of the texture you downloaded. Also, ensure that when you upload your web pages to your web server, you also upload the texture "file.jpg" into the same directory for the above command to work.
Alternatively, you can simply link to the texture using its URL which is provided when you click on the texture in the archive. Suppose that it's:
http://www.grsites.com/textures/pa/pa086.jpg
then in your HTML file, you include this tag, somewhere after the <head></head> section:
<body background="http://www.grsites.com/textures/pa/pa086.jpg">
replacing the URL with the correct one for the texture you want, of course.
Please note: if you want to use a texture that you have modified in some way (if you have changed its color, brightness, size, etc... using the appropriate functions on the web site) then the texture will only be stored temporarily on GRSites.com. The best way to link to the texture is to save it to your GRSites Folder. Once there, you can use the texture's permanent URL to link to it from your web pages.