I deleted it for you. You did not make a stupid mistake
Any mod or admin will be glad to help you out. That's what we are here for!
I am surprised that members can't delete and "clean up" their own galleries, but in that case, we can do it for you and don't hesitate to contact any of us.
I also moved your remaining picture to the Members Freshwater Tanks category. You had it in Freshwater Equipment pictures. On the drop-down box that you have to use to select a category for your image,the Freshwater Tanks and Freshwater Equipment categories are next to each other so it's easy to click on the wrong category when they are all in a drop-down box like that. You should be able to move categories yourself, but if you can't, again don't hesitate to ask an admin or moderator.