While signed into your HonestGamers account and viewing any page on your blog, click "Images" on the top bar to bring up a page listing any images you currently have submitted. You will have several sorting options, so that you can browse images in the order they were submitted, or alphabetically. You can click image previews to view the image at full size. Next to each image, you will find information about its title and submission date, as well as HTML if you want to include it in a post. Finally, the plain URL is provided, which you can copy to your clipboard and then paste in the appropriate field when submitting a new post to your blog that will use the image as its header graphic.
To submit a new image, just click the "add an image" link in the text paragraph at the top of the page. This produces a new form, where you will see image guidelines. Make sure any image you submit meets those guidelines. Click the "Choose File" button to select a file from your computer, give it a name that you will use to find it later, and then click the "Upload" button to finish. Your image should appear soon, and then you can start using it.