Click on Go advanced below the writing box, scroll down and there's a button that says manage attachments. Click that and if it's a file, click browse, find the picture and click upload. If it's a file from URL, copy and past it into the second box that says Upload File from a URL and than click upload.
Just a side note, make sure to clean the 1.5G at least two or three times a week 100% (taking everything out and cleaning it). Ammonia builds up very fast in small tanks. That's why we say 'bigger is better'.