The winner as of 8/31 is Fall Favorites, with 41% of the vote!
To update your signature* for the month of September, go to the upper right corner of the bump and click the Preferences icon (the portrait of a head). Select "Signature Settings" from the list. Once you've found your desired image (gifs are highly suggested), click the image icon (a mountain and a sun in a box) in the signature edit box. Paste the URL of the image itself into the popup window that will appear and your image should appear. To resize the image, click the Toggle HTML View icon (the two angle brackets <> ). Find the part that says '<img alt="" src="...' with the URL of your image. After the img, add width="300" then click the Toggle HTML View icon again to see the new size. You can add whatever number you'd like for the width, play around until you're happy with how it looks.
To clarify, this is (roughly) what your code will look like: <img width="300" alt="" src="https://media.giphy.com/media/10cl0BeEKI8Zu8/giphy.gif"> Remember to include the space between your width entry and the alt entry! Troubleshooting in advance: if the image is not being added, make sure you're grabbing the direct URL of an image (typically done by right-clicking and selecting "Copy Image URL"). Some sites will embed the image and make it difficult to directly link to the image, and others will block embedding with privacy settings (such as G+ and Facebook). In these cases, it may be easier to save the image to your computer and then upload it to a media hosting site yourself (such as photobucket or imgur).
*Unfortunately, signatures are not available in the app, but you can visit the Bump via your mobile browser and use your browser's options to "Request Desktop Site" to view everybody's signatures and/or update your own