If your not willing to pay for Adobe Photoshop( like majority of people) then I use Photopea( get ad blocker as the regular ads can sometimes affect the performance of the software for some weird reason), you can use that to create whatever advertisement you have in mind, then when it comes to uploading it, the server requires it to be an Imgur link, when you create the imgur link, right click the image and copy the link address and post that into the address link, that should work and then all you have to do is wait for it to be permitted by a Senior Mod+ after that you can use it with any image board, flags, billboards etc.