Try using a different online image hosting service! photobucket seem to stamp whatever with their watermarks which looks annoying, indeed! You can re-upload the original pic to something like imgur or even twitter(I forgot if they host them).
If you are to make a new one, most popular photo edit apps will do. There are also design apps that gives more option to banner and poster designs. Alternatively, you can draw one for your own, in that case any illustration app will do, such as Procreate and Adobe Fresco. (I used the last option :) )
PS. be sure to include the image's extension(.jpg .png)after the URL in BB codes. It may not display if you it didn't.