Im confused, i assume you want to create a gif, of a map you created, and upload that to your workshop page?

Yes. The principal image in my maps are empty and i dont know how put something there

The animation should be uploaded automaticly, maybe you need to enable the gif creation in options first, it has been a while since i uploaded to broforce workshop

Use the beta to make maps, it works better than the current build. Now it's too late for your already uploaded maps :/
EDIT : not sure you know that, but the game is supposed to make a gif of recording you playing/testing the campaign, but the latest update broke that :/